The irregularity of driving usually makes distance per day kind of useless. I drive the same commute to work five days a week, for example, but my driving on the weekend varies from nothing to several times my work commute, so km/day really doesn't matter. In the long term (per year perhaps) it might be more useful.
A much more meaningful term is cost per mile or km. My car costs about ten cents per mile to operate on average, so it's pretty easy to calculate how much a trip somewhere costs if I know how far away it is.