I do think these are useful. There is a wheel one too:
I used this self hitting technique to learn to really brush the ball:
In the video I am spinning forward rather than making the brush really thin and spinny. I should make a video of the slow spinny shots because that is what I really used the self hit method to teach myself.
But if you watch the kick on the bounce on my shots, you can see I am putting a lot of spin on the ball.
When I did a lot of practicing that, I would use a bucket with about 144 balls (1 gross). When I grab, I can grab five or six at a time and keep going till the bucket is done. You can really focus on the form of your stroke and the touch of the contact at the same time because the ball is just there.
The key detail is that I am hitting the ball on the second bounce which allows you to time the bounce more like if the ball was coming to you. Another detail, if the second bounce is low, like below net height, then you have to spin to arc the ball over the net. It is worthwhile practicing that.
But those spin balls are really good for developing your brush technique too. The advantage of the wheel is if your contact is too direct, you can't miss it. And if it is really too direct you can break your racket. So if you are not confident with your brushing skills, you would use a throw away racket rather than your actual setup.