As far as I can tell, and this is mostly just my opinion, the purpose of the context is to filter your list down into everything that you actually could do, at a given time.
So if you're sitting at work with a couple minutes to kill before a meeting, looking at "Buy more cat litter on the way home" doesn't necessarily add any value to your decision process.
It's a solution to a problem, which is that a complete list of all NAs for many people could be really really long -- too long to help you decide what to do in the most efficient way. A lot of the parts of GTD exist to deal with this problem, actually.
If you're the sort of person who doesn't have this problem, then you might not need the solution -- One Big List might work for you.
Cheers,
Roger