How is 得 used in 只有懂得节制欲望的人...？
In 懂得, 得 is pronounced dé and means "gain" or "obtain", as in the first definition:


  　1. 获取，接受：～到。～失。～益。～空（kòng）。～便。～力。～济。心～。


That is, 懂得 means "to obtain understanding".

The meaning you were referring to is a different use of the same character, and is pronounced de (neutral tone):


  　2. 用在动词或形容词后的连接补语，表示效果或程度：跑～快。香～很。


It is true that in your example, the 得 can be omitted without loss of meaning, but it is used to improve the rhythm of the sentence, as the emphasis is on the "懂得".