I am always amazed how we are constantly given reasons to exercise - to build muscle and lose fat. Its not always about appearances, it's about your health.
Once upon a time it was speculated that excess fat, which places more weight on the bones, could help build strong bones. Not the case any longer as researchers have found a strong (and positive) association between lean body mass and bone size and density. Contrary to prior belief, the findings found that fat either had no relationship or a negative one, meaning the more fat mass, the weaker the bones. What's more? Fat may also secrete substances that affect bone metabolism negatively.
So, we may as well exercise for our bones, especially since we're stuck with them for life.