In some cultures feet are considered to be automatically unpleasant or forbidden. Many Masters or Mistresses enjoy forcing their slaves to smell their feet. This provides feelings of humiliation for the slave, and power or control for their Master / Mistress.
Have you ever walked in something nasty, or come home from a hard day at work, removed your shoes and thought "Wow, I need a shower!" Now imagine being able to force someone to accept that. Force them to suffer your body odours as though it was the beautiful perfume from Paris. That is power. That is the authority and might of being a Master. When you can tie your slave, push them to the ground and rub your foot in their face, then there is no question of who has control. Your slightest will is their mission in life. When you can force a slave to suffer through your smell and still worship you, then your power is complete. Foot smelling is not just an activity, it is a whole state of mind and a form of total control.