Just in general there is a massive amount of research (like 100 years worth) showing calorie restriction in animals can have significant effects on things like lifespan. Also intermittent fasting has been shown to have serious impact in rat models on things like inflammation, cholesterol, obesity etc. So proponents of IF will say the act of fasting itself is making you healthier - but there is no evidence of this being the case for humans. tbf building the sort of quality human studies that will give clear answers here takes ages, and we don't have them yet.
As for IF and building muscle I don't really know tbh - I lift and just eat my dinner as far as that goes.