Using the auxiliary when it's not required if for emphasis, to make a contrast with a prescription "no." En español, se podría usar "sí" (con tilde): A-No tienes. B-Yo sí tengo....

"do" is very interesting because it's invisible (but always there!) in every Present Simple sentence. It becomes visible in negative sentences: I didn't have / I didn't see /I don't like... Anyway, to answer the question "do" - in the case of a sentence in the present simple - is used to insist that what were are saying is true. We can also use it to give strength to a statement. And for stylistic reasons. (Probably other reasons also) Example: - I live there in that house. - You don't live there! - I do live there! (we are insisting with "do") Example of giving strength to a statement: - I do like strawberry ice cream ! - I do love a good walk in the countryside! Example of a stylistic use: - I do believe we're speaking to an idiot. I hope it's a bit clearer to you. Unless you want to insist on something, you don't have to use "do".

I agree with the answers above but I would like to add something. Another situation you would use "I do have" (sub- to be - verb); when you are writing/speaking compund sentences. Example: I do have a gym membership, but I never go. She does have a car, even though she never drives it. Bob does like cats, unfortunately he is allergic to them. They do eat ice cream, but only during the summer. I am originally from California but living and teaching in Peru. Que tal!