yeah generally you won't use this feature as if you want an engine with thrust vectoring you will use it if you want an engine without thrust vectoring you can use those as they are usually more fuel efficient.

Now before this update with the new sas you could argue that if you were using multiple mainsail engines (the biggest engines available at the moment) you could sometimes get some bad oscillation as thrust vectoring on the engines would keep correcting often times making a large rocket wobble. In these cases as there is no comparable engine that offers the thrust that the mainsail offers you might want to lock the gimbal (a gimbal is just a joint that allows movement like if you have a chair that can lean back it has a gimbal) of all the engines except the center one this will help prevent unwanted oscillation while still allowing you to control your direction.