Those are like a suspension fork or shock on a pushbike or motorbike - rebuildable and you can tweak it unlike a sealed shock.
I've seen a bike mechanic friend rebuild a $1000+ Fox mountain bike fork and a long time ago I saw a KYB or Showa motorcycle fork pulled apart, cleaned and checked for straightness against a dial gauge on a slab of marble. After that, fresh oil and new seals before it was put back together and repressurized to spec.
On pushbikes, SRAM and Fox want you send it into them or QBP(the biggest bicycle parts wholesaler in the US) for service. Cleanliness matters on these.