Muscle is an amazing tissue of the body. It is the very part of our body that gives us the control that we want and need in life. The mind is a useless thing often if it doesn't have muscle to put thought into action. It is also a major part of our body's defense mechanism. It protects things structurally (joints, bones, etc.) and metabolically, and physically whether by means of escape or defense from the dangers both human and non that we encounter. I want to talk more specifically about these things that I have mentioned in order to help you to understand that it is good to gain muscle.
Protection is the first area to mention when discussing why you should gain muscle and is true in so many different ways. Muscles are major protectors of the joints and bones in our body. Strong muscles that span joints serve as braces for the forces that would otherwise separate and render these joints useless. The same goes for muscles that support long bones. If we didn't have muscles that were able to absorb the impact of forces that we experience every day than we would always be dealing with fractures and immobility. In fact the activities that promote muscle building are the very same activities that increase bone mineral density, which is especially important for women later in life after menopause. Muscles also help us to maintain our balance protecting us from falls. Muscle tissue is also huge regulator when it comes to blood pressure, and thus has indirect protection of many different major organs including the heart, brain, kidneys, and eyes.
Metabolically muscle is very important in maintaining a healthy weight and absorbing the harmful effect of the typical American diet. This is not to say that if you work out and gain muscle that you are automatically at a healthy weight and can eat whatever you want. But it does mean that those people who spend good amounts of time every week doing weight bearing exercises are less prone to things like diabetes, hypertension, cancer, heart disease, and atherosclerosis–all things that come with obesity–because muscle is a very good metabolizer of the extra fuel (glucose and fat) that we have every day in the typical "unhealthy diet."
Lastly for this article I want to talk to you about the importance of doing things that only having strong muscle will allow you to do. I am talking especially about your mental health here. Basically if you aren't strong than you likely aren't active (they go hand in hand) and if you aren't active than you are so much more prone to things like inactivity and depression its not funny. So take my advice and start lifting weights to gain muscle which is one of the most precious commodities that you have on your body.