First off, I would have to have 20 different wallets staking. Next thing, I would have to keep my computer turned on 24/7 365 days a year - that's an additional cost. Third, it depends from coin to coin, but if you don't buy a whole bunch of coins, you might not get a stake since other peoples wallets have more coins than your wallet. So even if I stake on my own, I might not stake anything and I'll consume the energy. Using a staking pool allows me to combine my coins with the coins of other people to increase our chance to stake new coins. The rewards are then proportionally divided among members of the pool. The pool owner most often takes a certain fee from the rewards obtained.

I hope this explains why a staking pool. :D