I thought about one on the VdlP last year.
Otherwise known as an 'ass pad' I think
I even bought three different ones!
Never took one and glad I didn't.
Just unnecessary added weight I think.
All those 'little' things add up.
There is usually somewhere to sit.
And if worried about sitting on wet/damp surfaces maybe take a piece of bubble wrap or similar ultralight material.?
Funny how different our needs are! Even when I don't camp, I always bring the mat, and I use it. Even on day hikes.
Especially if you walk in the colder season, it works well to put it on a wet/slimy with algae/covered in snow bench to sit on. The mat is also long enough for a second person to sit, or put the backpack on it.
It's also my picnic blanket and my "bench" when there isn't one.
Feet hurt? Need to rest? I lie down, and elevate my feet (on top of backpack) for a while. Take a nap even. Under a nice tree, next to a river... whereever and whenever I like to.
I use a shortened, foldable z-lite that weighs very little (300g). Have also used one of those very thin silver-foil/foam ones, which are even lighter, but not as comfortable.
Is it really needed? No.
You could also use a poncho as a blanket (but not when it's already raining and it's not really comfortable or warm on really cold days to sit on a poncho in the snow or mud).
Also, yes of course, on the Francés there are countless spots for sitting down (picnic areas, cafés). So, certainly no "need" for it.
But for me, it is still part of my essential gear. Makes me a bit more independent when it comes to stopping to rest for a while. No need to wait for the next bar or next bench...
I guess having walked a few other Caminos where there were often no cafés, picnic areas ect. within 10-20kms, I like to be a bit more flexible when it comes to this.
Most people would say "rest when you need to". But then there's no bench or no open café for the next 5 or 10km and what do people do...? They continue walking despite being tired, despite feeling that blister forming, despite that beginning pain in their knee... I prefer to just sit down to rest and have my second breakfast or lunch whenever I want to
.
Also, I'd be happy to sleep in any overflow or under a church porch during busy times. With the z-lite and the sleeping bag, I always have my bed in my backpack.
I make up for this "unnecessary weight" by not bringing stuff many others need, but I don't (like powerbanks). My complete toiletries kit only weighs 130-150g including towel. ect.
I guess you could call the mat a "comfort" item? I heard others bring rice cookers...