Talking about good club runner level, so 15-17 minute 5k, but I guess it applies to sub-elite/elites too.
How much can a runner gain from getting in a "good race" - doesn't have to be deep, if someone has run 16:30 before, ends up in a race with 2 or 3 runners at 16:15 and can latch on to that pace vs just running mostly solo after the 1st KM. The 16:15 runners might have had a sh!t race but have inadvertantly helped out other runners in doing so.
I've had this discussion with club mates, where there's been a couple of races this summer that people took 15-20 seconds off their PB by being dragged to a faster time - in subsequent races or other distances they get nowhere near the result.