I'm sure the person you asked has already replied, but if I may give my two bits:
- Bonus actions enable important design space. Sometimes, you want to make it so something can't be just used willy-nilly, but which can still be done alongside other, "big" actions. Healing is one of the most important applications here, since forcing a character to spend their whole action doing nothing but healing significantly discourages giving healing to allies (as, in most cases, all you're doing is putting things back in the state they were before, which merely delays the end of combat, it doesn't actually get the combat closer to completion.)
- Bonus actions, when designed cautiously, really can be quite simple and straightforward. There's a reason all three WotC editions have had them. 3.x invented them midway through and called them "swift" actions. 4e called them "minor" actions and used them quite effectively. 5e has...not used them as well as I would have liked (as is the case with most of its design decisions), but it is at least in the correct ballpark.
- Anything that attempts to replace them very quickly either runs into major issues, or ends up being effectively 17 different kludges that all accomplish the same goal. As an example, let's say we eliminate the Bonus action. Anything that used to be a Bonus action is now just an Action. But wait! Now that means there's a bunch of spells that are gonna be...well, kinda useless if you can't cast them as Bonus Actions. Healing word, for instance, becomes (almost) totally useless because you may as well cast cure wounds instead. (Technically, it still has non-Touch range, but that's it.) So maybe you say that healing word and misty step and other formerly-Bonus action spells now include a line that says, "When you cast this spell, you can also perform another action, but if you want to cast a spell, it has to be a cantrip." But....adding that line to every single formerly-Bonus action spell is just printing 20 extra words per spell in order to do exactly the same thing as before.
More or less, the reason we keep having these things is because trying to squeeze EVERYTHING a character can do into one single on-turn action ends up being a really, really confining, annoying design space. PF2e addressed this problem by giving every character three actions per round, so that different things could take up different amounts of actions depending on how complex and/or powerful the action is. And it isn't just tabletop that does this sort of thing. Video games, especially MMOs, have a "global cooldown" which effectively corresponds to the "one action per turn" limitation. But almost all games that have such a cooldown....also include abilities which are "off [the] global cooldown" aka "oGCD" actions, which the player can weave between GCD actions. WoW heavily uses this space for "trinkets" (basically accessories, akin to charges-per-day wands or the like), FFXIV invokes them all the time to add depth and gameplay engagement, particularly for melee-attacker type classes, and Guild Wars integrates not only that sort of stuff, but also the action combat "dodge" mechanics that exist entirely outside of cooldown stuff (having their own separate recharge).
This structure, where there are big "chunky" actions that Do Something Cool, and then smaller fit-in-between actions that add spice or focus or support, is extremely common because it's a straightforward, effective way to accomplish the design goals that most RPG designers are aiming for.