I also grew up hitting against a wall, and it is indeed a very good way to improve specific aspects of your game...however, a ball machine trumps a wall every time. Given all the things you can work on with a wall, a ball machine affords you even more versatility. Right now I have a few of the high school kids I teach working on their transition game. I set the machine to deliver a low ball with underspin, the kids will move forward, execute a half-carioca step, chip or slice the ball, at which point I kill the ball machine. If the ball lands in, I hit it back toward them--not with the intent to pass them per se, but rather to force them to hit a volley. As I'm striking the ball, they perform a split step, then execute the volley. Next student, repeat.
I've done that drill where the machine will feed the next ball to force a volley as well, but even though I'm using a slight variation, it's an example of something you simply can't duplicate on a wall. I can also set my machine up to simulate overheads - very high overheads (using a block to elevate the front of the machine), then set the 2-line function to random, forcing the kids to move from point to point as they hit overheads.
Again, you can certainly practice your slices, volleys, certain aspects of your footwork - quite a few different things on the wall, but the fact remains that everything you can do on a wall you can do with a machine, and a lot more.
That said, do you "need" a ball machine to improve? Absolutely not. In my prime I was a solid 5.0, and I never used a machine. I still don't now actually; I primarily use mine to teach with. But if you can afford one and you're serious about improving, it's a nice luxury to have.
One last thing - nothing takes the place of actual match play, hitting live balls; learning to read the shot, working on court positioning, anticipation, timing, spacing, etc. when hitting against a human being can't be duplicated with a machine or wall. A machine, like a wall is merely a training tool that allows you to hone specific skills. Live ball play affords you the opportunity to work on incorporating those skills into matchplay. ;-)
