When you repeat a word or phrase again and again at the end of a series of sentences, that's epistrophe. You can use epistrophe as a rhetorical device when you give a speech, to emphasize your ideas.
Epistrophe works especially well when it's used by public speakers who want to add emphasis and excitement to their speeches. Politicians use epistrophe all the time — one example is President Barack Obama's repetition, at the end of sentence after sentence, of the phrase "Yes, we can." As a literary tool, epistrophe is gives lines a dramatic emphasis and a repetitive rhythm. In Greek, epistrophe means "a turning about," from epi, "about," and strophe, "a turning."