I keep track of mileage for two reasons:
1.) I sometimes have goals in that I want to ride XX amount of miles or I want to exceed the mileage of a previous ride I did. My longest ride is 120 miles in one day. This year I want to exceed that.
2.) Using Strava it keeps track of the mileage on each of my bikse and I can use that data for maintenance such as tires, cassettes, chainrings or chain replacement and if I put up a bike for sale someone will inevitably ask..."how many miles are the bike?" Strava keeps track of the total mileage on each bike.