There are tools for developing players and developing optimal or proper strokes, (or to correct flawed strokes!) and there are tools for players to practice, drill and gain familiarity or comfort or confidence.
The latter drills would include hitting walls, ball machines, targets, baskets of balls, radar gun, Z-balls, etc.
The former would include a lot of the Joe Dinofer developed gagets such as the serve aids, the volley aids, and groundstroke aids. The Wrist-assist, Memory board, and things like the 8-board or swivil disks all can help in developing strokes or stroke elements.
For training new students, I like the PracticeHit Device, (a foam ball on the end of a fiberglass staff that ossilates when hit like a metronome, connected to a weighted base.)
And the reason I like it (and other devices like it), is because it takes away the directional concern of a shot and allows the player to focus on the technique, not the outcome.
When players are focused on where the ball is going, they will almost always adjust their technique to what they THINK will get the ball to the target. Yet, as most people know, learning proper technique usually does not equate into knowing where the ball may end up going until the technique--and subsequent aim--become mastered to some degree.
I've seen hundreds of players mistake activity for improvement. In fact, I've seen more people develop life-long detrimental patterns that contribute to their failure to advance from practicing technique that they THOUGHT was right (or were told was right by others!) only to find out later that they not only had been doing something wrong...they had been doing it wrong for a very long time!
Just a few thoughts on this topic!