hello dear,
It is not the verb, it is the auxiliary "do". And yes it is to emphasize the meaning.
If you read Mary Higgins-Clark - she writes wonderfuls novels - she often uses this means to emphasize what people are saying.
Let me give you some examples :
I DID see him at 9 when I arrived = I actually saw him at 9 when I arrived OR I am sure to have seen him ... OR It was him and noone else...
In this exemple you use the auxiliary of simple preterit to emphasize a fact occured in the past.
I DO believe he is innocent = I am absolutely sure he is innocent OR it is impossible that he has done that, it must be someone else...
I hope these explanations will help you dear.
Have a good day !
Maryse.