The difference, I guess, is whether you just want to get laid or whether you want to get laid by a certain provider. I guess that I've learned the hard way that providers aren't "a dime a dozen" -- there are real differences in quality amongst them (which isn't really such a stretch; it's a very personal service that they're offering, after all). So it's worth it, from that perspective, to plan ahead. It has everything to do with how you hobby, and what your expectations are. If you just want to get your rocks off spontaneously, and it doesn't matter with whom or the circumstances, then answering ads more or less blindly will most likely land you with someone. If you're looking for a session with a very specific type of provider, however, who offers very specific services (or a cut above the average), then, unfortunately, you have to make plans ahead of time. On the other hand, it takes quite a bit of the excitement out of it. Trying to find that balance is difficult -- which is why I brought up this topic in the first place.