What are the functions of mountains? I've heard that due to Isostasy that mountains stabilize the crust?

Answer 1

Mountains are a result of plate convergence. If plates didn't run into each other, there would be no mountains. Mountains do not stabilize the crust. It is because of isostasy that the crust is able to remain stable with the addition of mountains.

In other words, mountains are an effect not a cause.

Answer 2

I don't think they have a function really but because they are composed of less dense material than the mantle, they float above it and the oceans. They provide much of the sediment for the lowlands as well as the rivers. They are a consequence of plate techtonics.