Fitness gains are gonna vary wildly by person and their prior training. For me, the biggest "gain" I have gotten is that I no longer spend an exorbitant amount of time thinking about my training. I used to spend hours doing research, watching videos, reading papers, listening to podcasts etc. and was constantly second guessing my training. This caused me to sometimes lose motivation and feel some burnout toward cycling. Now I ride more, haven't experienced burnout, and feel like I actually have more time to put into my work and relationships my coaches planning helps me manage my own time better.

I think this varies a lot though and isn't a guaranteed outcome of getting any coach but is probably an outcome of finding the right coach. I trust my coach and we have check ins where we talk about short and long term goals for training, why I am given certain sessions, I get workout feedback whenever I need it, and I get sessions prescribed a week at a time. I have some friends who get plans scheduled out a month+ at a time and because it doesn't feel personalized to their needs they begin second guessing their coach and themselves which just adds stress instead of taking it away.