When I was mechanical, I used Trim to avoid chain / FD contact when cross-chaining a little more than I should.
I didn't need the Trim function during recommended operation.
Another way of explaining, when in the big ring, I could get to another larger cassette cog through the use of the Trim.
If you are cassette small cog and having to trim after changing to big ring, I'd say the adjustment is off.
Barry