@andraus Yes, because it significantly limits the amount of assistance you can get from the strap. You can also decrease or increase the assistance by adding or removing weight. It also gives you an idea of how much you are improving. 4 months ago I was able to do one with 8 kg assistance and recently I did it with 6 kg assistance, which gives me an idea of my progress to my goal.
However, if a fixed strap is already very difficult, that's where you should start.
Also if you are training for OAC, it's good to try different options like using a resistance band, which simulates the movement pattern a little better.
Anyway, FitnessFAQs did a video about the weight assistance method.