chris,
for liquidity. For example if you are trading intc,csco,dell or any such big cap, then you can put in an order on say 1000 shares and if it moves your way add to it up to 10,000 or 20,000 shares.
If it moves against you, you can get out quick with minimal damage.
When you trade something more volatile, the whipsaws can freak you out of the trade.
However that having been said, on smaller accounts when you are paying commiss per share you are often better off trading MSFT or KLAC since you can get bigger moves (30cents) on 2000 shares. You could probably make money in these trading only 500 shares. I use a 5 to 1 ratio so if I trade 5k cisco I will trade 1k MSFT. For me works out to about the same risk.
However for a beginner, The best is to start with something very liquid like cisco