Doctor O explains it well.
I recently have had a bit of experience. SME uses a screw on clamp that basically bends the record down on a precision cut cork mat.
The threads on my SME puck stripped and fell out a couple of months ago. This makes the record rattle on the precision cut mat, or without the spindle washer, the record does not go down to touch the precision cut cork mat evenly. Basically, a stripped puck renders the clamp/mat system kaput as a unified system.
I was never comfortable with the high forces used in the SME clamping system, and the stripped puck has sort of borne that out.
In order to get something working again, I put a piece of felt under the record, with the puck passive. I liked it a lot. It tilted the register more toward the midrange without challenging detail, and seemed to give a bit better bass resolution.
It also did not require the screwing down of the puck against the record.
I now use a piece of 1.5mm leather for a mat, and use a small felt circle under the passive puck. That makes it a "leather on cork" mat with a small felt damper for the puck.
I really like it, it makes everything sound more analog then ever.
However, I can't recommend a mat for SME turntables. SME is just too formidable in its reputation for precision, and one would breach its recommendations and customary clamping system with caution, but I am keeping the mat system.
There is also the possibility that anything "new" might seem better just because it is "new", but I have had the mat long enough to regard it as a keeper.
From what I have read about mats, there is no general rule. Every turrntable/mat configuration is unique, and requires experimentation. There is danger from both underdamping and overdamping, and puck/mat combinations seem to need to be worked out on an individual basis.