a) Emphasize the validity of a fact (usually stated immediately after the phrase) - e.g. "You done know they're running a scam."
b) Respond affirmatively to a query - e.g. Q: "Are you good for this money I'm lending you?" A: "You done know"
c) Express one's assent or agreement - e.g. A: "Boy, it's hot out today!", B: "You done know"
The phrase is also varied to fit the context of the conversation by changing the pronoun used (i.e. "She done know", "They done know", "I done know", etc.)
Jamaican dialect example:
"Yuh done know di gyal a lie 'bout 'ar panty size. Everybody know seh 'ar batty bigga dan dat!" (insert laugh here)