Sometimes I build commerce on my home colony so that I can get rid of junk in between exploration expeditions
Same. I also like it present to act as another source of fuel, supplies and crew on top of the waystation stock.
The tarriff is entirely irrelvant, as by the time you have a colony at all the 30% loss either way is utterly meaningless. Not once have I ever cared about it.
Commerce also provides you with a free 'preview' of various ships which have come out of your factories. Not guaranteed to be useful, but sometimes you'll find things there that are either helpful or interesting (in particular ships with beneficial D mods).
And none of this uses production capacity, it's just there.
Another thing you can use it for is rep-grinding with the independants. Buying/Selling a few thousand units of anything will get you a few points of rep, if that's something you ever need to do.
but I've modified the game files to change it to a structure in my game because it is 100% not worth an industry slot.
Also same. As much as I like the utility of commerce, the only way it's worth an industry slot is if you've already covered all your input needs. And that's not happening (usually) for quite a way into the game.
As a structure, I will 100% build it on my 'home' colony, and
maybe somewhere else if I have cause to visit it occaisionally.