Sūkarakhata sutta and the "arahants seeking benefits they already have"
www.palikanon.com has a clearer translation.


  The Buddha questions Sāriputta at Sūkarakhatalena as to why a monk who
  has destroyed the āsavas should revere the Buddha and his teachings.
  Sāriputta answers that it is because such a monk has secured freedom
  from the yoke. "What kind of security is it?" asks the Buddha, and
  Sāriputta explains that it is security obtained by the cultivation of
  the five indriyas of faith, energy, etc. S.v.233f.


An Arahant reveres the Buddha and his teaching not because he/she has anything left to attain, but because the Buddha and the Dhamma are what led him/her to Nibbana. The faculty of faith peaks out when a being attains Arahantship. The closer you get to Nibbana, the more respect and appreciation you will have for the Buddha and his teachings.

Incidentally, venerable Sariputta attained Arahantship by listening to the Dīghanakha (or Vedanāpariggaha) Sutta at the Sūkarakhata cave.