Upper Mounts (aka: Strut Mounts, Mount Bushings, Strut Plates, etc.) are attached to the upper part of a unit. On the front ones, they are usually visible when you open the hood, one on each side on the inner fenderwell. (in the rear, you may have to view in the trunk or pull the rear seat or inner body panel to see them).
Normally, these parts are reused when changing a unit. Like any automotive part, if yours are ok, there is no need to purchase them.
However, since they typically have to be removed when changing out the unit, many Customers change the mount at the same time as preventive maintenance, since they already have the vehicle apart and the old ones removed. This can also save big labor costs in the future if you have high miles on the vehicle and think one may go bad soon.
Front mounts typically fail first, since the rotate left and right when you steer. They take a lot more abuse than rear ones.
These mounts will work with OE units or aftermarket units, it does not matter. They are specifically designed to fit the vehicle.