In the Harry Potter books and films, we saw that Expecto Patronum is mainly used for tackling Dementors and Lethifolds. But what (if any) are other things that Expecto Patronum can be used for?
In-universe references only, please.
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Aside from being the primary way of protecting yourself from Dementors (and Lethifolds), the patronus charm appears to have at least two other canon uses, as well as several theoretical ones
On several occasions we see Patronus charm being used to communicate over long distances in a way that can't be easily intercepted, for example in Harry Potter and the Deathly Hallows:
Harry did not know where to begin, but it did not matter. At that moment, something large and silver came falling through the canopy over the dance floor. Graceful and gleaming, the lynx landed lightly in the middle of the astonished dancers. Heads turned, as those nearest it froze, absurdly, in mid-dance. Then the Patronus’s mouth opened wide and it spoke in the loud, deep, slow voice of Kingsley Shacklebolt.
‘The Ministry has fallen. Scrimgeour is dead. They are coming.’
In HP and the Deathly Hallows Snape uses his Patronus to guide Harry without revealing himself:
And then the source of the light stepped out from behind an oak. It was a silver-white doe, moon-bright and dazzling, picking her way over the ground, still silent, and leaving no hoof prints in the fine powdering of snow. She stepped towards him, her beautiful head with its wide, long-lashed eyes held high.
Harry stared at the creature, filled with wonder, not at her strangeness, but at her inexplicable familiarity. He felt that he had been waiting for her to come, but that he had forgotten, until this moment, that they had arranged to meet. His impulse to shout for Hermione, which had been so strong a moment ago, had gone. He knew, he would have staked his life on it, that she had come for him, and him alone.
At last, she came to a halt. She turned her beautiful head towards him once more, and he broke into a run, a question burning in him, but as he opened his lips to ask it, she vanished.
And off the top of my head
Since the Patronus charm can't be cast by a true Death Eater, you could theoretically use it as way of confirming that someone isn't one.
According to Pottermore, Remus Lupin is afraid to cast a Patronus as he...
...felt afraid that his corporeal Patronus gives too much away.
So, presumably you could use this as a way of determining whether someone is or isn't a werewolf
...those able to produce corporeal Patronuses were often elected to high office within the Wizengamot and Ministry of Magic
He jumped to his feet, his voice frozen in his throat, and raised Hermione’s wand. He screwed up his eyes as the light became blinding, the trees in front of it pitch black in silhouette, and still the thing came closer.
Well, it may be possible to use a Patronus to fight whatever potion Dumbledore drank in book 6. For Harry, the Dementors bring up his worst memories, and that potion that Dumbledore drank made him relive the death of Ariana, his worst memory. so maybe a Patronus would help to ease or erase the effects of that potion.
"the Dementor will feed on you long enough to reduce you to something like itself... soulless and evil. You'll be left with nothing but the worst experiences of your life."