I didn't think about this at the time and I don't know too much about developing games, but I know that modding support keeps a game alive. Just look at Fallout, Skyrim, and Left 4 Dead to an extent.
A good recent example is Teardown. The game had about 600 players daily and on March 4th when the steam workshop was added it jumped to 2,000+ players and 3,000+ the next day. Now there are around 1,800 players daily.
It would also mean less work for the devs. If someone made a good gun mod or something like that a