TOPIC:

I don't understand, how does it protect?

The piston axle. You put the sock inside the spring. On the piston axle

Something like this:

Yess

I see.. well good. But how can the shock operate then with the 'sock'. It will stiffen the movement.

That is only an option to care the shocks. You can use a plastic plate in front of them. These shocks were used especially in 1/8th gp buggies on muddy and dirty circuits.

Ok, then I understand. Swimming in mud will require a few rubbers around the shocks.

Just to keep them oil inside proof.

It's basically the same situation as greasing the UJ's (different topic), but this time the other way around.
The dampers are oil filled and the damper shaft going through the bottom of the damper body will have a thin layer of oil on it when leaving the damper body.

The oily shaft will be exposed to sand/dirt that will attach to the oil and then be dragged by the shaft through the damper body's lower seals. This will of course over time damage the seals, create leaks and the need for maintenance.

Adding the socks would reduce, if not eliminate, the amount of dirt in the damper shaft, and thus be a good thing for your dampers (and wallet)

I really need to relearn my english.....
Thanks Lars, I didn't think that the shaft was oily. But it is logic...