I have always known 得 as an adverb. For example 跑得很快 ，run quickly or run very fast. The following sentence
只有懂得节制欲望的人，才能享受到人生的真正乐趣
means "Only a person that understands (how to) control his desires can achieve true happiness." However, I don't understand the use of 得 here. I think I would have written it without the 得. Or is 懂得 a special phrase?
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".
Yes, you are right, “懂得” is a phrase which means “ understand”. And “得” usually is a structural particle , it is usually used in :“verb + 得+ adjective” structure, which expresses the comment or result of the verb, like: 跑得快 “快” is the comment of “跑”.
