There’s probably several reasons for warehousing: and it’s possible none apply to you (@bruce). At one point, and perhaps still, RW was “funky” in regards to remembering images included in a project. So if you moved your RW project to a different part of your hard drive, or later moved it entirely to an external hard drive RW would lose track of the images. This may be better now but I’m not sure.
Another reason is when you use multiple computers (as I do). And again this relates to the funkiness with which RW remembers images. When images are warehoused I don’t have to worry about working on my project file from multiple computers. (And, again, RW may have fixed this issue.)
A third reason for warehousing is multi-purposing images. If you know you’re only going to use an image on your website project then this isn’t important. But I often use my images in several places: web page, PDF documents, etc. etc. So warehousing keeps a consistent location for my images no matter what software/product I’m creating.