The answers given here are pretty obvious - that stop losses protect your capital. This is true for swing trading / holding positions overnight.
However, for day trading, if you are sitting there and watching your trades, I don't think stop losses are necessary. Personally, I don't put stops in the market, but I do have mental stops and have the discipline to stick to them.
The problem with stops for day trading is that if a block trade (or any other trade) goes off away from the current market, and it's below your stop (for a long position), then you will automatically be stopped out at the current market price, even though prices haven't necessarily hit those levels.
Believe it or not, block trades are quite common, so don't go around thinking that it's a rare event.