#1. Unnecessary = Not needed, not required, something extra in a negative sense, of little or no importance.
#2. Rewards = Gain, a payment, a return of sorts.
#3. Pleasure = A positive sensation; either physical or mental.
So, here we have a physical or mental feeling, or sensation, or emotion, which in truth, is an unimportant gain.
The writer is saying that the unimportant emotions we receive from pleasure is small compared to the many joys received when a person achieves a better understanding of spiritual matters.
Student or Learner