because it would serve no apparent goal that I value, but I've known individuals who wouldn't leave anything sitting out for more than a couple hours if it wasn't being actively used.

There are programs for behavior modification that attempt to optimize your cleaning/tidying time for you which recommend exactly this behavior. (My girlfriend did this for awhile.) It reduces one's need for lengthy cleanup sessions. So some of the people you observe doing this are serving a conscious goal.

Clearing one's plate may well be an optimization for populations that experience famine. In the US, there's little utility. In Houston, Texas doing this when eating out will almost certainly result in overeating.