The quick filter buttons at the top of the list are the perfect tool for quickly selecting (or excluding) certain types of tasks from the list you are looking at right now, for example the Next list.

(Usually, I suppose, the Tags are used for "minor contexts", and as you know I have already suggested in another thread that by allowing the "major Contexts" to also be represented in the quick filter bar, the quick filtering would become much more versatile.)

A very similar reasoning goes for Projects and Goals:

If you could set Tags for Projects and Goals, and have all of their tasks automatically inherit that Tag, and if you would then see that tag in the quick filter bar above the list, then it would become possible and very easy and quick to temporarily filter your Next list (or whatever list you have in front of you) and see only what you want to see right now from this list - without having to take the time and trouble to change to a complete project view, goal view, context view or to a saved filter view (no need to define and save temporary filters).

I believe very often a user would simply set such a Tag to be identical to the name of the Goal, Project or Context, just to simply make it possible to quick filter by these things also, but it would also become possible to assign other Tags, for example as a means to generalize and "connect" similar things that you often want to look at together (for example to view tasks from projects that you have tagged as "internal" or "controversial" or "risky", or view tasks that belong to a certain "family" of contexts.)