What incentives do professors have to increase the PhD completion rate in their department?

In other words:

How do professors profit if a larger (rather than smaller) proportion of entering PhD students in their department complete their degree?

(At my department at least, it seems like the only benefit is "warm-glow altruism". A professor gains no more from having more students at her department complete their PhDs than I do by having fewer stray dogs in my town getting killed by traffic. A professor's career prospects does not seem in any way affected by whether 80% rather than just 50% of PhD students complete their degree. Not surprisingly, the completion rate, at least at my department, is closer to the latter.)

Note: I am not asking whether it is a good idea to give professors stronger incentives to increase PhD completion rate. Neither am I asking for suggestions as to how PhD completion rates can be increased. Rather, I am asking what the incentives at present are (granting of course that this varies from place to place).