Moving over rough terrain works your suspension. Move over it fast and it really gives your spring system a workout. When it comes to those last precious inches of travel, it's always good to have a mechanism in place to stop the full compression before metal hits metal and you begin to damage hard parts.
Bumpstops serve this function. Stock bumps are typically hard rubber knobs meant to handle occasional suspension bottoming. But, if you're pushing your rig harder and using the full extent of suspension travel on a regular basis, you'll probably want to upgrade to some better bottoming protection for both driver comfort and to avoid axle and chassis parts damage. Aftermarket vendors offer choices to suit your wallet and your application.