It in common on Hallowe'en that people come to your door and say, "Trick or Treat', but why do we give sweets?

I had a thought that, on Hallowe'en instead of giving them a treat, I would give them a trick, so I could show them a card trick or maybe a disappearing act but I wouldn't give them any sweets. Why don't we do that instead of wasting money on a packet of sweets we originally got for ourselves but had to give it away to some poor child who wants so gummy bears!