Curious: moderator servicing?

... someone selling a fatboy in the classifieds. In his ad he mentions "... only needing to service every 3k rounds ... " or something to that effect regarding moderators in general. 

?? Why? For real - just curious. Periodically clean out / blow out residue, dust, other build-up that may accumulate inside of them? 

First time I've seen reference to servicing a moderator.