It emphasizes the statement if you ask me.
Only many years after learning of the speech I read online that it's allegedly "a gaffe".
(In case you put the emphasis on "ein", you would clearly emphasize that you are just one of the many Berlin citizens.)
"(Lui) è un dottore", "(Io) sono un dottore".
"Un" is the indefinite article.
Subject is usually dropped, thus the brackets