Bitcoins are ordinarily only ever created in chunks of 25 at a time, with the whole 25 paid to a single person. Furthermore, the race to get the 25 BTC prize in a given block is highly competitive.
If you set out mining on your own, it may be a long time before you can make a return. Pooled mining allows you to receive smaller, more frequent, steadier payouts instead. If you have a slower computer, or a CPU miner, then pooled mining may be the only way that you will ever mine any bitcoins at all.
