FS will be helpful with roots; arguably not essential, but helpful. Sand, maybe not so much (you've mentioned in the past wanting a bike to ride in the sand).

Certainly, you can set up the suspension to match your style of riding and the trails you typically ride. Some bikes allow you to essentially turn them off. I can turn my rear suspension off, and some people like to do that for long climbs or any period where it's not needed, as it lessens pedaling efficiency. Your bike's front and rear suspension may have additonal settings that you can use.