Why is 'desservir' to clear and to service at the same time?

Bonjour Fiona !

Here it's one of those weird cases in French where usage has rendered a prefix meaningless in some contexts.

In the same vein, you also have "rentrer" being used to express "to come in" (entrer) as well as "to come back". 

I hope that's helpful!
Bonne journée !

Bonjour Fiona !

I had a look at the questions linked to that lesson, and all of them include a hint with what verb to use :)

Bonne journée !