Sure, you can ground yourself, but many other things can be static generators in the typical environment. A mat is there to protect sensitive parts when putting assemblies or chips down on the work surface or reworking components.
I used to work in telecommunications equipment repair and believe me, we used ESD mats in addition to personal grounding straps (wrist or foot based).
A good ESD meter can also be helpful for your work space.
Is it overkill to put in an ESD mat or have a meter nearby? Maybe, but it is a one-time investment and my personal experience is that is worth it.
I like Desco products. The make a good, quality ESD mat.