You don't need them at all, but if you want and you are happy spending the money or don't have the wherewithal to make something yourself, then why not.
A milkshake will do fine esp on short runs or a glass of water.
Chicken sandwhich, scrambled egg on toast, peanut butter on toast - all good post work out food.
I like PLJ myself though will have a chocolate milkshake after a long run.
Oh and bananas. 
Edited: 12/05/2011 at 08:46