When a catcher sets himself up, he’s doing things that just might influence your delivery, 60 plus, feet away.
A solid, squared posture, where the shoulders are squared to you, and the kneecap protection is squared and in line with the mitt, offers an excellent mental image in your mind for whatever pitch you’re about to deliver. Stabilizing this image in your mind offers a consistent targeting process that will reinforce your command in so many ways. One of the best stabilizing images is the mitt, squared with the kneecap protector. Any mitt position below that location, enhances the probability of a pitch in the dirt. Pitches in the dirt are a GO for runners on in many cases.
I also want to add, some backstops adopt a side step posture, where the entire body is off to one side. This may be a preferred body posture for some - but, this body posture is easily susceptible to a pitch that’s down and in or away and in the dirt. A pass ball has a high probability in this case. So, if your backstop prefers to slant squat, be mindful of pitches called for that are down and in or away. Any loss of control on your part or your battery mate can mean extra bases with runners on and another set of pitch counts.
In addition, controlling the ball in the mitt, foul tip, and avoiding a pass-ball, stability with controlling the play of “dropped third strike”, is just in there for good measure.