Fasting reduces blood pressure, cholesterol, and blood sugar. It stimulates autophagy, which is the process the body uses to recycle old or malfunctioning cells. And it stimulates the production of human growth hormone.

All of those effects either cause or are associated with a wide variety of positive health outcomes. Fasting is very good for you. And the benefits of fasting increase with prolonged fasting.

As far as muscle mass goes, the body does not consume muscle until the store of body fat is almost depleted. Now, if you just sit around while you are fasting, your muscles will start to atrophy. But that is due to inactivity, not the fasting per se.

I have a reasonably muscular build (I lift twice a week), and fasting like I do has not caused any muscle loss at all.

From an evolutionary standpoint, our bodies are designed to go extended periods of time without food. That's how we've survived as a species over the millennia.