Why did McGonnagall ask Filch to find Peeves before the Battle of Hogwarts?

Question

Extract from Harry Potter and the Deathly Hallows:

The aged caretaker had just come hobbling into view, shouting "Students out of bed! Students in the corridors!"

"They're supposed to be you blithering idiot!" shouted McGonagall. "Now go and do something constructive! Find Peeves!"

"P-Peeves?" stammered Filch as though he had never heard the name before.

"Yes, Peeves, you fool, Peeves! Haven't you been complaining about him for a quarter of a century? Go and fetch him, at once."

Filch evidently thought Professor McGonagall had taken leave of her senses, but hobbled away, hunch-shouldered, muttering under his breath.

Is there any explanation what did McGonnagall need Peeves for?

Answer 1

The next time we see Peeves, he's attacking the Death Eaters by dropping Snargaluff pods on them from above.

It's pretty reasonable to assume that McGonnagall made Peeves aware of the impending assault and asked him to assist in the defence of Hogwarts.

Even as they stood braced, looking for the opportunity to act, there came a great ‘wheeeeeeeeeeee!’ and, looking up, Harry saw Peeves zooming over them, dropping Snargaluff pods down on to the Death Eaters, whose heads were suddenly engulfed in wriggling, green tubers like fat worms.

‘Argh!’

A fistful of tubers had hit the Cloak over Ron’s head; the slimy, green roots were suspended improbably in mid-air as Ron tried to shake them loose.

Answer 2

Peeves is the biggest troublemaker in the history of Hogwarts, since he is a poltergeist. She obviously wanted him to wreak havoc among the Death Eaters.

Answer 3

Peeves lives to do one thing and one thing only. Causing chaos and havoc with pranks and mayhem and shenanigans. As is demonstrated multiple times in order of the Phoenix with various staff members resisting umbridge's BS, the staff is well aware that giving Peeves any kind of permission to do his thing is tantamount to aiming a weapon. Given that we know that peeves is probably intelligent enough to tell the difference between students and death eaters, the decision to use him makes perfect sense. Mcgonagal is going to give peeves permission to do what he does best. Make other people's lives hell. Which he did.

During the Battle of Hogwarts in 1998, Filch (albeit forcibly and only after he was persuaded by Minerva McGonagall) invited Peeves to take part in the defence of the castle. Peeves delightedly caused chaos among the Death Eaters by dropping Snargaluff pods on them. After the battle, he was heard singing a victory song to celebrate winning the battle.[10]

Found here: http://harrypotter.wikia.com/wiki/Peeves

Answer 4

Here's a fun tid-bit: Peeves came into being due to the collective mischievousness and chaos from every student to pass through Hogwarts. The students are both his lifeblood and essentially his parents. So while he'd normally just drop dungbombs or water balloons on them, if they come under attack, he'll personally erase the threat if he can. The reason noone kicks him out of Hogwarts anymore is it's literally impossible; he just gets more and more powerful over time, and isn't truly a danger, so they just gave up.