Purchasing "faster" RAM may improved the performance of a machine, but it will certainly not have the magical effects that many inexperienced users expect from it. I think they feel that adding RAM is like some kind of special elixer that will somehow fix all the problems with their computer and magically prevent them from having to perform a real hardware upgrade.
As a rule of thumb, if you understand the concepts behind what makes one stick of RAM better than another, then you are more likely to appreciate the difference it makes. If not, I wouldn't recommend reading to deep into it.