So there just aren't that many folks of the type you're talking about on the male side. A few more on the female side but then sub 20 5k for a lady actually sits pretty well.
I think we have a skewed perception on here of what certain 5k times are worth. 21:42 gets you in the top 10% at my local Parkrun where Parkrun as a whole is probably a better standard than the country (UK) as a whole.
So you're asking is it worthwhile training lots to be in the top 10% or so of a local run / race. I can see why folks would think it is. If you're not a young male then that's putting you even further up the chart on an age grading basis.
But back to the actual question... yes, it probably is pretty decent training for them. Until you're doing about 10 hours a week of aerobic work, then you've probably got room for more gains there. At 8:30mm, that gives you roughly 70 mpw. Potentially diminishing returns after that.
What may be of benefit (particularly if they're not improving) is to work on biomechanics. Plenty of folks who have never done track work or drills or any of that jazz, simply don't know how to co-ordinate their limbs to move *fast*.
That's not necessarily even that much to do with mile repeat style speedwork. More strides and fast last 400m building up to a fast last mile of runs just to practice the muscular co-ordination they need. Generally not needed for folks who have come from other sports.