I'm not native speaker))) but recently I have learnt this grammar on my English courses. Well, do\does\did are the auxiliary verbs and in such sentences (you wrote) use to show emphasis in a positive sentence. With the present \ past simple, add do\does\did before the main verb. With other auxiliaries stress the auxiliary verb. For example: I do like opera = I really like opera; I did close the back door! = I really closed the back door! She does love you = She really loves you. After did and does you don't need end "ed" or "s, es", but without them (do, does, did) you need ends. ;))))))))

They are using the word do to emphasis the sentence.

we used a verb do in sentences for confirmation ..when i said i do respect you ...i confirm for my respect for you

Because it emphasises that you DO, as opposed to that you DON'T.

As the others have said it is for emphasis. In England especially we will say 'oh I do like that book' meaning I like this very much, a little like Americans say 'that is so...' with the emphasis on so.