I think this is probably a more popular idea these days, but if you're not convinced, let me bring up a few points that might change your mind and encourage you to strive for muscle growth. First and foremost, muscle is an expensive tissue. There's a reason why it's challenging to gain muscle. Therefore, your body is a survival machine and only wants to have as much muscle as necessary. Expensive tissue? What do you mean by that? Well, the more muscle you have, the more calories you burn without effort. Meaning that in order to keep the muscle on your body, it costs calories to sustain. Think of it like this: what burns more gas, a four-cylinder engine or a V8? The V8 right?
The same goes for our bodies. The more muscle you have, the more daily caloric expenditure you will have simply because it's "expensive" to hold on your body. Now, there is some controversy behind the gain muscle to increase caloric expenditure mentality. However, I do think there is something to be said here. People I listen to say it's around 30-100cal per pound of muscle gained, but there's also plenty of opinion articles about how it's not that high at all and is more like 7-13 cals a day.
Regardless, the anecdotal evidence seems to point towards people with more muscle having an easier time managing weight. This could be for a variety of reasons like more balanced hormones and possibly healthy user bias, but it's hard to flush out. However, because of its high cost, it's also hard to keep on your frame. This is why doing resistance training is so important. It's your way of telling your body "hey, we need this stuff". Remember, our bodies are survival machines, and they don't want to hold onto anything that it deems unnecessary.
Okay so we've loosely established that it's easier to lose weight or maintain a desired weight with more muscle mass, but anything else? Hell yes! If you have a focus on muscle gain, it is likely that you will be doing movements that also encourage muscle growth in areas that aid our everyday lives. People with weak knees can make strong knees by growing the muscles that surround the knee. Same with any joint. Growing the muscles around the joints allows for the joints to take on less of the burden of stress as the muscles are now responsible for the loading instead of the joints themselves.
Ever hear of Wolf's law? If you haven't, it's the reason why resistance training is oftentimes recommended for people with osteoporosis. When bones are put under stress, they increase in density. This is why astronauts have to consider this even more than we do because they lack of gravity alone will cause their bones to atrophy. Using weights to resistance train will long term help your bones become stronger due to this simple law.
Anything else? Again: hell yes! Having a focus on muscle growth also implies that you'll need to be eating a diet that requires some sort of basic healthy thought behind it. Why? Well because you'll never grow muscle without a proper diet. Alright, one last thing: having a focus on muscle building generally will lead people to think, "how can I maximize my efforts" which tends to drive us in a direction of general health like getting more sleep. Sleep is massively important as this is how we recover and having a focus on muscle growth will hopefully encourage those who don't normally consider sleep to start thinking about it.
Overall, you could probably read this and scoff and think, "yeah okay, meathead, we get it," but I really do believe having a goal of muscle building should be in everyone's back pocket. Keep in mind that according to the National Academy of Sports Medicine, resistance training 2-3 hours a week can completely negate over 30 chronic illnesses later in life. So, if there's any motivation to put resistance training at the forefront of your life, there it is.
Certified Personal Trainer
University of Connecticut for Biomedical Engineering