Each bitcoin node performs the important task of verifying transaction and block chain data.

For example, if a powerful miner, one day, decided to produce a block that gave himself 100BTC, all the other nodes on the network would ignore that block, because the block reward is currently 25BTC.

Further, running a node helps the network by providing users access to bitcoin data.  If we do not have nodes on the network that accept incoming connections, it would be nearly impossible for anybody to send or receive bitcoins.

Each additional node on the network helps strengthen bitcoin.  Each node is a "vote" for good data.  If we have too few network nodes, an attacker could potentially control the data we see.