Simmons is an athletic guard who can't and won't shoot 3's, it's pretty much the rim or bust for him.
Bertans is a 6'10 forward with deep range from 3, shutting him down mean you have another player attached to his hip 23-25 feet away from the rim. If he's playing with Green; that's two players that opposing teams can't sag off, 3 if Leonard's on the floor too. That's a lot of space for the Spurs to operate in, not to bring up Bonner but it was the primary reason why the offense was usually always great with him on the floor.
He seems to be able to attack closeouts, which is also useful but it's almost impossible for teams to consistently shut down a shooter, without opening up something for a offense to exploit.