When DO is used in an ordinary declarative (positive) sentence it expresses emphatic assertion, usually in contrast to a prior negative. In speech it receives primary stress.
Jill is too shy to work well on teams, but she does work well on her own.
Speaker A: Jack doesn't work here any more.
Speaker B: He does work here, but he's on the night shift.
In older English the DO forms were employed more freely, alternating with simple present and past. You will find it frequently in Shakespeare:
O she doth teach the torches to burn bright!
Why, man, they did make love to this employment.
But it had largely disappeared from the Standard dialect by the 18th century, and survives today only in dialects.