From one point of view, the Touch Up mechanic does seem redundant. After all, increasing quality also increases the probability of obtaining an HQ result.
However, the chance to increase HQ from simply increasing Quality is a relatively small increase, as well as a (in my opinion) overly simplified HQ mechanic. I'll give an example, using the mooglebox HQ data:
Quality of 300 yields a 1% chance to +3
Quality of 400 yields a 2% chance to +3
However, each Touch Up gives a new independent chance to +3
Quality of 300 with 3 Touch Ups yields a ~3% chance to +3 (1 - 0.99^3)
This mechanic makes the synthesis process more a balancing act, attempting to achieve high quality without sacrificing too much durability. To my mind, this brings an element of skill (consideration of balancing opposing needs in an uncertain, probabilistic environment) into the synthesis process. With the current material / part HQ process, it is much more repetitive, simply a process of using careful synthesis as much as possible, and repeating the synthesis many times.
Additionally, the Touch Up mechanic for ingredients would add more value to the use of HQ dropped / gathered materials. Whether this is a good thing or not -- I couldn't say.
If the Touch Up mechanic were retained, but used for making ingredients, the current probabilities for +3 result could be retained to achieve HQ result.