past tense - What is the difference beetween "You decided" and "You did decide"

I know that "You decided" is simple past, but I've never ever seen this kind of construction until I listened it in a song (You did decide):

Pronoun + Simple Past(Past assistant did) + simple present

Then, what is the difference? Does this construction really exit?

Answer

In simple sentences, we don't usually use the verb to do to indicate the tense(past/present). It only occurs in a question(interrogative)

did you decide it? -past

do you decide it? -present

However, you can use it in simple sentence for emphasis

You decided it. - simple past tense

You did decide it. -simple past tense but more emphatic.

EDIT: It also occurs in negative sentences

I don't decide it

I didn't decide it