The type of leg lift you're discussing is definitely for measuring glute strength. Are they asking you to do it to build strength, or was it just part of
testing your strength? I've done it loads of times to test my strength during initial and final PT visits (mostly for my back/hips), but not commonly for my knees....
With that said, it IS great for building glute strength...but ti depends what the goal/purpose is. I've had to do it for years on my left side, even if I didn't just have knee surgery (though that makes things worse and guarantees starting all over again due to recovery and weeks of not doing my usual routine) .
I have an issue with maintaining hip/sacrum alignment due to a car accident in college. The ligaments were horribly stretched, and the hip was out of alignment for months (I was a pour college kid with no medical insurance in the US).
Unfortunately, after every knee surgery, my "back" PT twists me like a pretzel to realign things, and that exercise becomes a daily chore, adding weight over time. It works wonderfully, but again, if they're asking you to do it after knee surgery, there are many other ways to accomplish the same goal. Keep in mind my Back PT makes sure one
always does it correctly, as many people do more harm to their back if this is exercise isn't done properly. Too much lift/twisting often causes L4-L5 vertebrae problems/pain. I've been there and done that after having my son, as I wasn't taught the correct way before I found this specific physical therapist.
So yeah---it's common as a test, but not so common as an exercise to do unless you've lost A LOT of strength over time or have a preexisiting back problem.