Has the nuance that the thing you don't have to do is something you don't want to do in the first place, and by being able to avoid it, you're happy because of it.
Taken from Book 3 (Week 9, Day 2) of the Jet Translation and Interpretation Course for 2008-2009.
Remember that this can also be translated as "...and it's good that..." See Unicom N2 Dokkai, pg 20. The reason "sumu" works this way is because the Chinese character's (original) meaning is "to relieve.".