You people play this game in a really convoluted and overengineered way. Just swing at the trash/boss one/few more times. Or spend that boring buff-up times on something else - like learning boss patterns (and parrying).
As in almost any other souls game, they are useless. Literal checkboxes on development spreadsheet.
There are exceptions - e.g. Nioh 2 somewhat forces you (in underworld depths) to not only use elemental effects but also cycle through them as with each use the boss is gaining resistance to it. And you have to apply two effects to cause confusion which allows you to damage the boss in any meaningful fashion (while that debuff lasts).
But these are exceptions, not the rule.
Before the “Breach” update focus potions were useful to test runes in town. Now it’s just house + bed + hp->focus rune.