I started noting daily mileage on a calendar, eventually moving over to weekly desk calendars so I could add a short description of each ride.
Then one year, "working" the week between Christmas and New Year's, I decided to put it into a spreadsheet. For a while I played with various graphs (accumulated annual miles, weekly miles, monthly miles and rides, etc.); they're generally useless, but sort of fun to watch.
Recently I started recording my rides on Strava (to skew the results of some fair weather bike commuters who take some amazingly circuitous routes to avoid city streets I consider eminently suitable for cycling), and Garmin Connect (to hook into weight loss apps). About the only thing I use those for is to see if I'm keeping my pulse down to a reasonable level, but there's lots of data (not to be confused with information) in those databases.
I also track mileage for major maintenance actions (mostly chain and tire swaps).
Why not all the other stuff? I'd rather ride than record. Take average speed for example: I'd have to note whether it was a recovery ride, intervals, or endurance ride; climbs, group or solo ride (and sometimes the splits), wind, temperature and clothing in the winter. I don't get enough useful information out of a record to make it worth while to write it all down.