Curious as to why you believe it does nothing for a GK? Futsal is played at a faster pace so a GK is developing their reaction time. Since they need to make quicker decisions they are working on recognizing danger quicker. Probably the biggest plus is footwork. Todays Keepers better be able to play with their feet under pressure and Futsal teaches that.
Now all of this is my opinion and yours is obviously different...
I have to agree with GKDad. I wouldn't go so far as to say it doesn't do anything for a GK...2 pluses are the reaction time is much faster in futsal because the shot is usually taken from closer, and it teaches keepers to play with the feet under pressure. But because of the floor and the compact space, I think there's additional risk for injury and also the keeper might develop some bad habits. In particular, field keepers are taught to not block with their feet, to only give up the corner if it can't be caught, to look up for the aerial shot, and to reduce the angle fast on a one v one. Futsal keepers must block with their feet, the corner isn't as dangerous, the angle isn't as severe due to the size of the playing field and the goal, and the ball because of its weight doesn't get as much elevation. My GK son sometimes plays futsal but as a field player...wouldn't want him to play as a GK in futsal.