Why use 的 at the end of a sentence such as 我一定會去看他的?
"的" in this case means "certainly", "really", "I am sure that..." as the conclusion says in your question.

For me, such sentences are the same.


  我会去看他的。
  我一定会去看他。


You can say "我一定会去看他的". The mood sounds stronger (I think it's not much stronger), but I can't tell you how strong it is (this is a natural language, not math). I would use this when I want to express that "please don't remind me of it any more. I am sure to do that. I won't forget it". For example:


  Mom: 别忘了早点回家过年。— Don't forget to come back home early for the Spring Festival holidays.
  You: 知道了，妈妈。— I will, mom.
  Mom: 一定要早点啊。— make sure [you will come back] early!
  You: 我会的。— I am sure I will (mom, you don't need to repeat that. I will. I am sure.)


Also, you don't need to treat "的" seriously. It's used very common.