Why do paints and primers say "Apply second coat within 1 hour or after 48 hours"? What's happening during the interim?

The specific durations vary (sometimes within 2 hours or after 72, for example), but it seems to be pretty consistent with paints, primers, polys, etc.

I assume it's got something to do with the curing and basically you should apply the second coat while it's still tacky or after it's fully cured, but why? What happens if you apply the coat after, say, 10 hours?

submitted by /u/VagabondVivant
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