I understand the concept of a context to be the physical environment needed to perform an action. Furthermore you are also supposed to consider time required, energy level, and priority; which in the GTD book are defined as part of the overall context. The book mentions that all of these together determines your actual context at the time. Of course he does not recommend that we mange our lists that way, because he also wants the system to not get unmanageable, or require sophisticated software.
And yet these are useful ways to parse out contexts that are available in a given place.
When I'm low on creative energy, I'm likely to fall into my "Calls" or "Research" contexts. Both are available darn near anywhere (research is 99% of the time online, and I have a cell phone and my full address book's in my iPod), but neither really belongs in the "Anywhere" context.
Yes, research could fall into @Computer, and calls into @Anywhere, but I find it useful to split them out because they are, to me, a real change of internal state, even if I'm physically anywhere.
Also, each one gives a rough picture of necessary time. Research is typically 15 minutes +, and calls are more often quickies, 5-10 minutes tops. (Which is why longer conversations end up as @Agendum instead)