Foot licking is very popular both as part of the BDSM scene, and with regular sex due to the way feet are sensitive to light pressure (like tickling).
By itself, foot licking can be very pleasurable for the Master or the Mistress, however in BDSM play it is also sometimes used as a form of humiliation as the slave is forced to kneel and serve their Master / Mistress. This remains true when the Master or Mistress demands you kneel and lick their shoes or boots.
Masters or Mistresses should ensure that their feet are reasonably clean before requiring foot licking.
Because of the pleasure involved or the fact that foot or shoe fetishes are quite common, slaves may also receive this from their Master or Mistress.