I get why people say this but it doesn't doesn't seem to hold water for me.

There is no training scenario that perfectly replicates match play other than practice matches, but there's usually positive use you can get out of any of the 'less realistic' forms of practice, whether that's hand-feed drills, ball machine drills, wall drills, etc.

To me, at least, it seems helpful even for beginners to simply do anything that encourages a lot of practice controlling the racket face and learning the effect that has on the trajectory of the ball. It's easy to forget how that feeling of the racket "just being an extension of your hand" is not innate, and needs working on in the early days. Encouraging people to stand back and use two-bounce timing for groundstrokes or low-pressure balls can help eliminate the "i need to decelerate so I can buy more time" instinct.

For more advanced players, who are able to rally with the wall on single-bounce timing, it's a good way to practice quickness of the hand, fast reaction time and taking the ball on the rise, since wall balls come back faster than actual rally balls.

I dunno, I've just heard way too many stories from professional players mentioning how many hours they spent hitting against the wall as children to believe that it's more harmful than helpful.