I got the explanation from Utah State University http://www.usu.edu.
“But I bet you’re wondering ─ or at least I hope you are ─ why Latin would have a nominative singular for “I” and “you” at all. Aren’t those pronouns embedded in the verb? Why would you need to say ego, when amo [I love] and video [I see] by their very nature indicate first person singular? Good question!
“The answer is these pronoun forms like ego and tu are emphatic. They’re used to emphasize the subject, not explain what the subject is in the way nominative personal pronouns function in English. In English we have to say “we” if we want to indicate that “we” is the subject. Yet, that same information is embedded in every Latin finite verb. So, the Romans didn’t use their nominative personal pronouns to explain what the subject is; rather, they used them to emphasize it. For instance, if we said in Latin tune amas? “Are you in love?”, in this case, the Latin speaker would be emphasizing the subject, “you”, by including the Latin nominative personal pronoun tu.”
In the graphic below, replace noun with pronoun.