I keep track of mileage for two reasons:
1) So I can show people (namely cousins who proudly ride the "Coal Train" and other community members who see bikes as only annoying recreational machines) that bikes are actually economical and useful.
2) So I can prove to myself that I am capable of pushing myself and can discipline myself to get out of bed an hour earlier in the mornings.

Mileage total (commuting only) for the year: 1,354 miles
Car miles saved: 2,372 miles (this is more than the bike miles because wife and I share a car, and there was quite a while when she needed it for work, so she would have to drop me off at work and pick me up at the end of the day, thus riding bike saved twice the amount of car miles)
Gallons of diesel fuel saved: 74 (We drive a diesel VW Jetta that gets ~43 mpg, if we had a less fuel efficient car then this number would be much higher)
Money saved based on average fuel price for the days I biked: $190.09 (Diesel fuel is also at the lowest price it has been since 2008, so this would be higher if the price actually reflected the cost)

The bike was a gift, so every penny saved is another penny in our bank account (and another penny I can spend on the bike and bike-specific things)!

I have only been biking for just over a year (I started December 15, 2014) and I didn't track any mileage before March, so everything is actually higher, but I don't feel like doing math to guess how much higher. I'm an internal accountant by trade and do enough math at work.
I also use my bike to work as training for Cyclocross. I did three races this year, and plan to do a full season next year, maybe some road races as well.

Today will probably be my last commute. My RD cable either pulled out of the STI or it snapped (haven't gotten a chance to look yet), but I might zip-tie the RD and go singlespeed for a couple of workout commutes. We'll see. I don't know when I will have time to fix that.

PS: My average speed has gone from 10mph last December to ~ 17 currently. Not that this matters much, but since I am using it as training this is a happy thing. Plus, I have 500' of climbing in my 7 mile commute, and generally have 20-30 lbs of clothing, food, groceries, work stuff in my panniers when I ride. So I feel pretty good about that.

PPS: Also think about the savings in Insurance costs for a car, and the purchase cost of a vehicle. If we were to buy a very cheap new car ($2,000) just for me getting to work and insurance on it (assuming the same $630 we pay for our current vehicle), that is more savings of $2,630. I did just get a motorcycle (for free from a brother) though... Insurance on that was $300 I think? But still, big savings!