In my view, categorization is simply the grouping of items with common characteristics. Contexts categorize by "Resource", i.e., places, tools, people, etc. I use Contexts in 2 ways:
1. Reactively - if I am in a place or have a tool or person available and I have some time and energy/interest and it is of sufficient priority, I can zero in on a particular list of items that become Doable in the Context I am in and pick something off. But I never prioritize a "convenient" item ahead of an Urgent/Important items that might require a different Context.
2. Proactively - I can plan my schedule for efficiency by using the Context lists to combine items that can logically be Done together. If my day isn't filled with appointments, I can create my own appointments with @Car or @Online, etc. and structure my day.
Since on most days, I have few appointments and a lot of control over my Calendar, I use Contexts for daily planning more than for Doing "in the moment".