RE: Why track, I think it's for both:

* I want to be able point to past achievements and see the value I'm adding (both for personal well-being and also CV writing/interviewing)

* I want to be able to see what I've done in a day/week/month/quarter to see if I'm performing well and delivering what both I and (potentially) my manager expects.

* If asked why X hasn't been done yet, I want something concrete to point to so I can provide my reason, without fumbling around for excuses or reasons I can't remember.

Is there any reason you chose RTM specifically? I happen to use Bear for my daily notes, but I'm wondering if perhaps just a to-do tracker (Kanban, RTM, whatever) and a single achievements.md file might be better.