Perhaps most importantly, splits simply draw attention to a stock from people who might not have noticed/known about it otherwise (all publicity is good publicity and such).
It also makes it cheaper to buy in even numbered lots - thus transaction costs, depending on your brokerage, may be slightly lowered.
Splits should also double the volume, which makes a company more likely to pop up on charts and (hopefully in the not too distant future) most active lists/alerts. Added volume will usually mean added volatility, which will probably mean more option interest (which can be good and bad for shareholders).
Splits probably affect margin/short interest trading too in some ways... Just some ideas off the top of my head... However, I'm no expert by any means...