An individual image may not be that big, but start adding layers, stitching, working on multiple images, running other things in the background, and it all starts to add up. Just run Ps and watch how much scratch space it's using for a given image - way more than the base TIFF if you start doing things to it.
You can also look at using extra RAM for a RamDisk and do things like put your Lr Catalogue on it (yes, there's a risk of losing data if you crash before closing and having the RamDisk write-out, but it's super fast!