I use an excel spreadsheet. After a ride I sit there cackling like a mad man entering in stuff that possibly has a purpose other than satisfying the inner nerd.
Sometimes it's nice to see how far a particular bike has gone, or how much wear I got out of a set of tyres. It's less nice to see how far I'VE gone (it's never as much as it feels).
I've also found it useful in planning trips when I know I've done the run before so I'll look back on the log to see how long it took me.
But the awful reality is that it's all about the inner nerd who likes to cackle like a mad thing over numbers.