> it's actually specifically for server-rendered content and using it for front-end development doesn't actually add much

It's for server-rendered content that can be used to build a frontend app. I know because I've done it several times.

> I would also have to write custom JS for the functions I would try to invoke with htmx

The difference is with invokers you would have to re-implement everything from scratch and with htmx you typically only need to implement some parts that it doesn't handle, like eg listening to a DOM event and doing some action in response.