I track my mileage, mostly as a way to motivate myself to put in some extra miles some months, because I could certainly use the excercise! If I know when I replaced something (like I've got an e-mailed receipt or I remember what month I replaced it), then it's easy for me to figure out how long a part lasts, since the vast majority of my miles are commuting, so I can figure that per month or if something lasts longer than a calendar year, I know exactly how many miles it got during that year. Gives me an idea of what to plan for. For instance, I now have an idea how long shifter cables last in the conditions I ride in and my riding style. Helps me keep a stock of spares on hand when I need it if I have some idea how long something is going to last too.

To each there own though. The first six years or so I rode, I didn't track my miles, and probably would've laughed/scoffed at the notion I try. Of course, since then, it's gotten a lot easier to track your miles. Since I've started commuting again after a several year hiatus and then started tracking my miles, I've appreciated the extra motivation to get out and ride more on my days off, if I am shooting for a monthly or yearly mileage goal. This is actually what started me on seeking out routes that weren't the shortest/easiest between work and home, and my commutes have gotten a lot nicer now that I'm in the habit of exploring or taking longer routes to put in more miles. Since I'm not always taking the same route, it keeps my shortest routes fresher and gives me variety. When I started tracking my mileage, I wasn't expecting the benefit of knowing how long parts lasted, but I've gained that from it too. This is yet another thing I have to thank Bike Forums for, if there wasn't a sticky thread here where people log their miles, I probably never would've started. Thanks folks!

I can see how some folks wouldn't be interested in tracking their mileage, but I don't find it onerous (kind of fun actually) and I've gained a number of unexpected benefits from it.