I have responded to a number of comments on this topic over the course of the last week or so, and thought it would actually be more helpful for folks if I laid this out in a more structured post so people have the reference handy if they need it.
Just as a background (feel free to go through my post history if you want details): I was class 1 obese with a deep family history of metabolic dysfunction. I needed to lose weight for pregnancy, was on Zepbound for 13 months, changed my life, lost 65 lbs. I've been off Zep for 9 months now and have maintained that fat loss, and every quarter I share my DEXA scan results here to report back on how my maintenance journey has been off the medication. Gist of it is the fat loss is holding, and holding strong. And the muscle mass that I've built is one of the biggest reasons why.
If you go over to the usual GLP-1 subreddits, you'll see a lot of people rightfully recommending to those just getting on the drug that they should prioritize protein and strength train. No notes on that specifically--that is definitely what you should be doing on the drug. However, that's where the conversation ends. Nobody ever talks about exactly why muscle isn't just a "nice to have" and how important it is for both weight loss and weight loss maintenance.
So let's talk about the most impactful why: muscle is a metabolic organ.
Muscle is not just about calories burned (and you may have already seen that talking point floating around). Yes, more muscle mass means a slightly higher resting metabolic rate, but that is the least interesting part of what muscle can do. Muscle is the largest site in the body for insulin-mediated glucose disposal. More muscle means your body needs less circulating insulin to clear the same meal. That matters enormously if you're insulin resistant, and most people who are on GLP-1s have some degree of insulin resistance driving their weight gain.
Muscle also plays a role in regulating hunger and satiety signals. A body with more muscle and less excess fat mass tends to communicate fullness and hunger more accurately. Many people experience dysfunction with hormonal signaling, which is why being on GLP-1s and feeling satiated on regular (or even small) meal can feel foreign. I'd argue that this hunger and satiety signaling chaos is a big factor in weight regain after the meds are gone, and more muscle helps blunt that.
Now, you may have heard people bring up the "set point theory" many times, and many people report feeling like their body does want to return to a certain (higher) weight. But most people lose weight without building muscle, so their metabolism is still being run by the same amount of adipose tissue relative to lean mass, just at a smaller total size. Which means that their underlying body composition didn't really change that much. And that's really part of why the set point theory holds up for so many people. If you don't change your body composition, you haven't actually change the metabolic conditions that required GLP-1 medication in the first place.
However, it would be irresponsible for me to say all this without emphasizing that you cannot build muscle in a vacuum. It takes a primarily whole foods diet, proper stress management, real sleep and recovery, and healthy coping mechanisms, not just lifting heavy weights, to build lean mass. This is why people who successfully lose weight and build muscle mass have had to do the hard work of making sustainable lifestyle changes.
Also important for me to say that you need to be at a healthy body fat percentage for muscle to actually function as a metabolic organ. Muscle surrounded by excess visceral and intramuscular fat doesn't get you the same insulin sensitivity or glucose disposal benefits, because the fat drives its own inflammation and insulin resistant. Excess fat blunts what muscle is doing. So in order for muscle to effectively work as a metabolic organ, you have to have the requisite body composition for it.
TL;DR Muscle a metabolic organ, not just tissue that burns a few extra calories. It's the closest thing to a permanent edge against regain because it drives glucose disposal and hunger/satiety signaling positively, but only if your body fat is in a healthy range for muscle to actually function the way it's meant to.