If you're going to be lying down on the ground for an extended period of time, a shooting mat with padding makes a lot of sense.
You won't shoot well if you are uncomfortable. I shoot long range black powder cartridge rifles off cross-sticks from a prone position and anywhere up to 80+ shots in a day at some matches. I sure as sh!t don't want to tough it out without a "proper" shooting mat.
A mat serves two purposes, 1, it provides padding and comfort
and 2, it keeps you and your gear away from the crap on the ground.
It can be anything from a simple high density foam camping mat to a purpose built shooting mat. Whatever floats your boat or is within range of your wallet.