The simple reason is that it allows you to work with more images simultaneously. Most systems will come with at least this much RAM now. But the more RAM the merrier I always say, and the faster you will work. Remember, nobody ever returned a computer because it was too fast. Time is money, especially if it is your own time or your own money.
More memory means fewer crashes and more productivity.
Images and their manipulation will hog RAM and lock your system up if you don't have enough. Adding RAM is almost always the most cost efficient way to speed up a system. The best thing about a computer with lots of RAM is that you will probably be able to put off buying a new computer longer than your friends. I use 256 megs at work in my new system, but I got along just fine with 160 in the old system. RAM each for the system to deal with them at a meaningful speed.