I must admit, I was confused by the relevance when I first came across the function - I suppose you can't have the whole screen 'green' - that'd be daft, so there must be an rule such as: "50% of all terrain is higher than this altitude" for instance, so you get an idea of where the highEST bits are around you (relatively speaking).
Of course you alwys know that you can descend to the 'highest' altitude safely (taking into account FMC position error etc etc), but if you're over a 'black' bit you could descend to the 'lowest' dsiplayed altitude safely.
Not much use that I can immediately see apart from that, and, as you suggest, to give you general situational awareness of the surrounding geography.
Flying with yellow & red bits on the screen is where the extra terrain awareness is really appreciated, of course, which is what the system was primarily designed for.