Here's a silly question. Why buy a tool for something a bike shop can do for you in 5 minutes and that you generally only have to do once?
Well, you answered your own question, but even if it's a job I'll only do once, I like to be able to do it myself. And I love any excuse to get new tools. Plus, while I do buy from my local shops, I happened to get my last fork via eBay, and I felt really self-conscious bringing them something to work on that I didn't purchase there.
I've found that I have a $50-ish cutoff, where if the tool for a particular task costs more than that I'll take it to the LBS. So for example I've got the doohicky that sets the star-fangled nut, but when it comes to putting new headset cups in, I'm not paying $115 for the headset press (I'd probably make one, in that case). Various bottom bracket tools? Check. Thing that faces the BB threads? Heading for the shop...