Optical quality, color correction, contrast, better mechanicals.
If you are going to be shooting mono then there is not going to be much of an advantage paying for a top quality APO when you can get a decent mid range ED or “Semi-APO” that will pull in the same photons. I use a “semi-apo” 6” and get what I think is good data. Granted my Lum and Blue channels have some bloat but when I am shooting SHO it does not matter. Now don’t get me wrong I have a nice AT65EDQ and it puts out much crisper OSC data and I don’t have any bloating in Lum or Blue but with the $$ you save with a non super premium 6” you can buy a lot of other goodies and more scopes too. At one time I had my smaller 65 on the 6” and I shot the color with the small scope and the narrow with the bigger one. Anyhow like all things in this hobby there are many different paths to get where you think you want to be and ultimately whatever you pick there will always be other options and tweaks to get you to that next level. Obviously for visual a top APO is hard to beat.