Field position still matters even in OT. If you punt from the 50 and you pin them inside the ten, it's worth it. If you get a touchback, it's not worth it.
The value of the decision depends on the outcome of the punt. 4 and 2 is no gimme, and its value has to be compared with the relative values of a punt inside the ten and a punt to the 20.
Which is worth more do you think? 20 yards of field position or the winning points?
Saints were on the Niners 40 thereabouts. They had 2 options - go for it, or punt. They punted and got a touchback (odds of downing it inside the 10 from that part of the field are low, so that's to be expected) which means they gained 20 yards of net field position.
Their other option was to go for it. 2 yards to keep the drive alive. If Payton could go back, which do you think he would have picked?
Overtime rules are such that once you get past the 50 on your possession, punting it is tantamount to quitting on the game because once the opposing team gets the ball back (usually at the 20) the only have to drive 60 yards to win.