The use of comma in written English
The main reason to leave off/on commas in this kind of sentence is to make clear whether the "which" clause is a specifier or not. When I say
The horse, which won the last race, still seems winded.
I am simply supplying additional information about the horse. On the other hand,
The horse which won the last race still seems winded.
Means that I want to be specific about which horse I am speaking.