Just like tires on a car pads only have "X" amount of miles before they begin to lose their ability to seal a tonehole effectively. Over time pads compress, causing leaks to develop making the pad less effective. Assuming the player has been regularly cleaning their instrument, a pad can be "floated" or adjusted to seal the leak. If the pad is overly worn or if there is a buildup of dried spit and calcification on the tonehole, the pad may not be able to seal properly. Certain notes will begin to sound more airy than full, or may not sound at all. This is when a pad will need replacing.
Cork is used on most instruments because of its sealing ability, its flexibility, longevity, and sanding ability. We use corks for clarinet tenons and mouthpieces, saxophone necks, flute headjoints, and many other pieces for regulating keys. Cork wears over time from being compressed or dries from age. Both of which will require cork to be replaced.
This only pertains to wooden instruments; Clarinets, Oboes, Bassoons, piccolos etc. The wood is still alive and needs a certain amount of moisture in order to keep the wood from drying, shrinking, and cracking. Wooden instruments need to be disassembled, cleaned, and bore oiled to retain its moisture and prevent cracking. This service only needs to be done every 3-5 years, assuming the player is regularly swabbing their instrument and is seen at least once a year to check for overly dry wood.