In my limited LaTeX experience, I've always found that I can accomplish what I want by just using the relevant set of packages, even for more complex formatting. If I were to try and create a configuration for someone else I would be hard-pressed to justify creating a class file; instead, I would just create a config.tex file that includes the relevant packages and commands and then tell them to just \input it in their main file. However, I'm sure that there is some additional flexibility afforded by creating a class, so my question is: When is it actually worth writing class files?
EDITSome further clarification of what exactly I'm asking. My question was more meant to address the case where a more specialized class is called for, such as the one in Alan's answer. In the case of, say, maintaining a thesis template for a university, what is the advantage of creating a custom class instead of just using packages? The beamer example is a good instance where the entire layout is different, but in that case what we're doing is creating an entirely different template. In the thesis case, though, I would guess that the base class does a LoadClass of article (or something similar) since we're generically constructing something pretty similar. To that end, I'm trying to understand why that approach would be preferable to the package-based configuration approach I stated above.