I bought a bike computer when I got back into cycling a few months ago. I decided that I wanted one with cadence, although I really couldn't have said why. I had the notion that it would be useful to know how many RPMs I was doing.

I have found it to be really useful - although I agree that you need to work out gears by feel for example, I found that the cadence feature helped me to work out when gears were too high and also, conversely, when they were too low. After a while I didn't need to look at cadence and went by feel but it was useful initially.

And it's still useful to know when I'm slacking off