Needs - things that are needed to run the layout correctly. Could be anything from one 14XX and auto trailer, or a dozen LNER A4s if you are modelling the ECML
Wants - things you like that don't fit in with the above scenario
ISTR that David Jenkinson did a series of Railway Modeller articles on the subject in the 1970s called something like Rationalise Your Stock Building. The conclusion was most people have too many rare locos because they like them and not enough of the ordinary locomotives such as 4Fs.
Jason