Preloading an image in javascript can be useful if you're using one image several times on a page, as if it doesn't load you'd be left with a lot of horrible red-X's (or whatever). Preloading solves this problem by ensuring the image is loaded.
Not being a Dreamweaver user I can't be certain, but there's every chance it actually uses JS for it's rollovers, just hides it from you under a nice interface. Then again, it may use CSS (I do believe it's been converted to feature CSS extensively in newer versions) which would make them quicker to load & accessible to more people (not all people have JS enabled).