And sure, when you hear a cry for help in the city, you do have the opportunity to drink a potion. And find you really didn't need 23 Strength to catch a pickpocket who stole an NPC's coin pouch.
There are absolutely times when players know ahead of time what they are getting into. And just as many times that they don't. Not every group has the benefit of Pass Without Trace or a stealthy owl familiar or a wild shaped Druid or invisible Rogue to scout ahead- if you don't have darkvision, you own perception is at disadvantage/-5 passive to locate enemies trying to hide from you (and if you have light, well, enemies will notice you coming from a mile off).
Enemies built for ambushing are very good at it, and some foes are so good at disguising themselves that there's not even a mechanic to know that they aren't some terrain feature or ordinary statue.
And if you do down a buff potion with a long duration, like an hour, there's still no guarantee you'll get to use it for multiple encounters. You might drink it, claim it's benefit, and then find out the party really needs a short rest.
So the most efficient way to ensure you don't waste consumables is to wait until you need them- at which point, action economy might still prevent you from using one.
Then there's active DM misinformation- sometimes a villain goes out of their way to confuse the party about what they are about to face. Subverting expectations to create challenge is a time-honored tradition, after all. At which point, you'd be very foolish to use resources prematurely.
Without any dedicated monster knowledge rules, even if I have cold and fire resistance potions, there's no way to know if the lich we just ran into is going to open with fireball, lightning bolt, cone of cold, or synaptic static until it's too late- in a situation like that, you could down all your buff potions before the fight and still end up wasting precious resources.
I know some groups approach dungeon crawls like a group of Shadowrunners. But it's been a long time since I've seen that kind of play attempted or even supported- go look at the recent threads about encounter pacing to see all sorts of DM advice to force players to not be cautious or careful in order to preserve the balance of the intended 6-8 encounters/long rest play loop!
It seems to me that doom clocks and timed objectives not only discourage taking rests, but also discourage careful scouting and buffing strategies.
I find this entire tangent to be somewhat confusing. "Players don't need bonus action potion use because they should be using potions before combat".
Well then, if they're using potions before combat, then they don't need to use bonus actions to use potions, so what does it matter?
