I'm guessing that for most people the tiny grants available on this site are not going to make the difference between being able to do a project and not being able to do it. So why bother with tiny grants?
Because here, now, being given money to do something is a signal that someone else values our skills. It shifts our sense of what it is we're doing, even if the amount that changes hands is tiny. It turns a small act we might have done anyway into accredited proof that we are an artist. And we're all artists!
Because it helps to motivate us by creating a sense of obligation. It can (I hope) push people from daydreaming about doing something to actually doing it.
Because it creates connections between people who might not have met otherwise.
But mostly, because I find it amusing. It turns something serious (looking for funding, or offering funding) into a form of play, a game and a performance that anyone can take part in.