The Patronus Charm might have involved some seriously difficult magic, but it came in seriously useful on a few occasions. And not just for defeating Dementors…

When Harry (unknowingly) cast a Patronus against Malfoy, Crabbe, Goyle and Marcus Flint

The first time Harry successfully cast a Patronus was during that Quidditch match against Ravenclaw. After weeks of hard work with a Boggart-Dementor that forced him to relive his mother’s death, this incredibly advanced spell suddenly came surprisingly easily to Harry – which was obviously because, as Lupin later informed him, it was not Dementors he was fighting, but Malfoy and his cronies:

‘You gave Mr Malfoy quite a fright,’ said Lupin. Harry stared. Lying in a crumpled heap on the ground were Malfoy, Crabbe, Goyle and Marcus Flint, the Slytherin team captain, all struggling to remove themselves from long, black, hooded robes. It looked as though Malfoy had been standing on Goyle’s shoulders. Standing over them, with an expression of the utmost fury on her face, was Professor McGonagall. ‘An unworthy trick!’ she was shouting. ‘A low and cowardly attempt to sabotage the Gryffindor Seeker! Detention for all of you, and fifty points from Slytherin! I shall be speaking to Professor Dumbledore about this, make no mistake! Ah, here he comes now!’
Harry Potter and the Prisoner of Azkaban

As Patronus-casting exercises go it’s not life-saving, but it did help Harry’s Quidditch team secure an epic victory. We think that counts.

When a time-turning Harry’s Patronus saved them all from the Dementors

Harry’s actual first Patronus was more impressive, even if it did initially cause some confusion – Harry saw his future self standing on the opposite bank; briefly thought he was some unlikely reincarnation of his own Dad; nearly suffered the Dementor’s kiss alongside Hermione and Sirius… well, it was all a bit of a muddle.

That is, until Harry-from-the-future (who, it turns out, really does look like his Dad) saved them all by successfully casting an actual Patronus, which scattered Dementors like skittles before ending with a bow.

As Dumbledore later told Harry: “‘Prongs rode again last night.’” So that’s that cleared up.

When Prongs saw off the Dementors in Little Whinging

By Harry’s fifth year he was a dab hand at the old Patronus charm, having also used it to charge a Boggart during the final Triwizard Tournament task. But nothing prepared him for the appearance of Dementors near the Dursley’s home in Little Whinging, the summer before he returned to Hogwarts. And yet, there they were, ready to leech life out of any human who crossed their path — even if that human was Dudley Dursley.

Handy for Dudley, then, that Harry was such a dab hand with the old Patronus charm:

He had run barely a dozen steps when he reached them: Dudley was curled up on the ground, his arms clamped over his face. A second Dementor was crouching low over him, gripping his wrists in its slimy hands, prising them slowly, almost lovingly apart, lowering its hooded head towards Dudley’s face as though about to kiss him. ‘GET IT!’ Harry bellowed, and with a rushing, roaring sound, the silver stag he had conjured came galloping past him. The Dementor’s eyeless face was barely an inch from Dudley’s when the silver antlers caught it; the thing was thrown up into the air and, like its fellow, it soared away and was absorbed into the darkness; the stag cantered to the end of the alleyway and dissolved into silver mist.
Harry Potter and the Order of the Phoenix

Rescuing Dudley’s life also led to a thawing in relations between Harry and his cousin. Not that Harry realised at the time, but what greater way to show thanks than a nice cup of tea? Even if said tea was cold, left on the landing, and delivered by Dursley two years later. It’s still better than nothing.

Every time anyone from the Order of the Phoenix used their Patronus to send messages

Patronuses weren’t just useful for defeating Dementors – in a world where there was (apparently) no such thing as encrypted texts, they also provided a foolproof method of communication. Tonks used hers to let Dumbledore know Harry had been safely located on the Hogwarts Express, following his run-in with Malfoy. Kingsley Shacklebolt used his to let everyone at Bill and Fleur’s wedding know the Ministry had fallen. And during the Battle of Hogwarts, Professor McGonagall used hers to call the rest of the Heads of House to Harry’s aid.

Because who wouldn’t want to receive a personalised message delivered by a shining silver animal? Much more attractive than a DM, if you ask us. Not to mention a lot more reliable.

When Harry and Hermione’s Patronuses led Muggle-borns to safety from the Ministry of Magic

When Harry, Ron and Hermione entered the Ministry of Magic in their Polyjuice-Potion-enabled disguise, it was good news for those summoned to stand before the Muggle-born Registration Commission. Even disguised as a thoroughly mean wizard, Harry couldn’t stop himself from rescuing others:

Harry whirled round, throwing off the Invisibility Cloak; down below, the Dementors had moved out of their corners; they were gliding towards the woman chained to the chair: whether because the Patronus had vanished or because they sensed that their masters were no longer in control, they seemed to have abandoned restraint. Mrs Cattermole let out a terrible scream of fear as a slimy, scabbed hand grasped her chin and forced her face back. ‘EXPECTO PATRONUM!’ The silver stag soared from the tip of Harry’s wand and leapt towards the Dementors, which fell back and melted into the dark shadows again. The stag’s light, more powerful and more warming than the cat’s protection, filled the whole dungeon as it cantered round and round the room.
Harry Potter and the Deathly Hallows

And for Mary Cattermole and the other Muggle-borns about to plead their case, Harry’s Patronus was a life-saver. Along with Hermione’s (slightly less corporeal) Otter Patronus, Prongs led the Muggle-borns out of the dungeon. Which was perhaps not the most discreet way to escape but, as always, you can’t fault Harry’s commitment to the rescue attempt.

When Snape used his Patronus to lead Harry to the Sword of Gryffindor

But of course, it was not just Harry whose Patronus saved the day.

With Harry and Hermione despondent after the events in Godric’s Hollow and Ron trying desperately to find his way back to them, it was Snape’s Patronus – the Silver Doe that was somehow familiar to Harry, perhaps because it recalled his mother’s own Patronus – that brought them back together.

It also led Harry to the Sword of Gryffindor, without which they may not have been able to destroy the Horcrux-locket at all. So, pretty handy all round.