A few sessions ago the party I DM came across some Gnoll lookouts in the forest. Three trivial Gnolls against that party (7th-8th level in a 1e-variant game) should be a complete pushover - and was; but it still had to be played out because had any of those Gnolls managed to flee and alert the many dozens more just over the hill (and by extension, alert the whole complex the party are there to explore) it would have made a rather big difference to how the rest of the adventure plays out.
The players didn't know this at the time, however; they just took out three Gnoll scouts and carried on.
All of which is to say that the characters - and thus players - can't tell a meaningful battle apart from a triviality until (sometimes well) after the fact, meaning they all need to be played out in full in order to not risk giving meta-info the players wouldn't otherwise have.
I've also had seemingly-trivial wandering monster encounters turn into unexpectedly big deals for another reason: the party start asking why that "monster" is where it is, backtrack it to its lair, and turn that into the adventure rather than whatever it was they were doing before. Bandits and brigands - every time.