I used to think the same way. Recently I started my own consultancy business, and the first thing I did was write a small shell script to track my comings and goings - to the second, so that I'd have good billing data. It has saved me an amazing amount of bookkeeping, and I regret not doing this twenty years ago.

Previously, I spent I don't know how much time going "now, how long did I actually work last Monday?" whenever I've tried to figure out whether I've worked enough or not (there was, of course, a target hours/week in my contract when I was employed). Now I know, at the push of the button: I worked eight hours, sixteen minutes and three seconds. OK, then!