I ride in for a few reasons. In no particular order, they are:

Time on the bike - I would spend 20 minutes commuting by car in the morning. It takes 25 minutes to commute by bike. On nice days, it's just a bonus to be able to ride in the morning and in the evening.
Health - It keeps me active, gives a little exercise
Money - Each day I commute I save a little less than $2.00 on gas. It doesn't seem like much, but three days a week for a few months starts to overlap in full tanks of gas

I don't ride in the rain, or when it's too cold. But when it's sunny, I'll usually make the commute, for the reasons above.