Imagine, that you use multifitting tool somewhere in PvP region with miserable trade hubs. You missing some modules. Eve offers you ‘buy now’ function but you can not buy what you need in Jita (for example) cause it is different region.
I suggest , that in case, that your multi fitting tool finds out, that you miss something, it will offer also new button : “Add to ‘buy later’ list”. So you can semi fit few different kind of ships, fly to better trade hub and use your “buy later” list to get all modules you need with one or two clicks.