I find the meaning of "Go over" is that "to move or travel towards someone or something"; so, what is the difference between "go over" and "go"?
Usually very little difference.
In this context, "over" is an adverb. If you say "He went over to his friend" it adds the sense that his friend was "over there", it emphasises the change in state. For example you might use "go over" if you "go over to your friends house", since his house is a different house to yours, so there is a change in state. In many cases the difference between "go" and "go over" is very small, and usage is determined by idiom.
There is also a phrasal verb "go over" which means "examine"