I use the turntable lock for Prince racquets with O-holes, Head racquetball racquets with string channels and Wilson racquets with rollers. When I am asked to put a Python grip on a racquetball racquet, I mount the racquet on my machine and lock the turntable then as well.
Use the locking table whenever you feel it helps you.
Some depends on ease of table lock of the machine.
Years ago my old Serrano machine never had a table lock' so was never needed back then.
After a break from stringing and starting again a number years ago, with a new machine that has a great lock, as just pull knob in and its set, push in and its unlocked..
It was so easy to use, I was using it to lock table even when weaving. Now I just typically use it on O port racquets, tying knots and weaving last 2 cross strings, as I find it easier to weave those last 2 cross strings with table locked, especially with nat gut as last 2 cross strings has tendency to twist, and it just takes a little more time to weave those, as I either push a loop, or weave one at a time like you are stitching so string does not twist at the end.
Agree with @jim e ease of use is a big factor
my newish prince.5000 has a easy table lock, as descrived above
my first dropweight machine, klippermate, had a manual removable 3" lock pin;, I would use it to lock the "table",, PITA,, I used it very rarely..
Only you know if you'd find the turntable brake useful or not. Some do, some don't. Many people have a brake on their machine, but don't use it.
On one of my machines (happens to have glide bars), I have a brake, but never use it.
On another machine (happens to have swivel clamps), I use the brake for all sorts of things (mounting/unmounting, tightening knots, final straightening of strings, and when using a calibrator). However, I never use the brake (on any machine) for stringing O-port/EXO3 type frames. Why? That's another conversation, for another time.
I don't understand why so many people say they use it when tying knots. I only use the lock on the annoying Prince O-ports/Speed-ports/whatever. Other than that, I find it unnecessary, although some people might find it useful in other situations.
I use the table lock for O-ports and sometimes when trying to get string through a blocked hole. When my stringer isn't in use I lock the table and put the cover on. Why? No clue. Just have always done it.