If you've got a couple of pieces or more of outboard processing gear (verbs, EQs, compressors, etc.) that you want to easily chain together - or seperately - to any recording or playback channel you wish in any order you wish and you don't want to mess around with a spaghetti bowl of cables to create those chains, then you are in the market for a patch bay.
If you have to ask whether or not you need a patch bay, then you probably don't need one.
G.