A player with a good forehand slice is annoying to play against. It's a very dangerous shot, b/c it is unusual and players aren't used to dealing with it, which leads to a lot of UEs. Alternatively players will often hit a relatively safe or spinny response to avoid hitting a UE.
It is thus often utilized as a point reset.
Santoro famously used his 2hander to bait and switch opposing players, often daring them to come to the net very much like the reverse of the federer dying cc bh slice. This is particularly effective against guys who hit very flat shots.
It is also used to crack the rhythm of a power hitter.
totally agree i have two great tennis buddies that can't match up with me baseline to baseline, then they discovered i have a problem handling the forehand slice. they give it to me out of nowhere. totally screws up my rhythm. and they usually do it when they are on the defensive when i give them some heavy ground stroke shots. the one advantage now is i have learned to read this and force myself to go to the net more. which is awesome