A Gore endorsement of Dean would be huge, ofcourse. Not only would it help Dean's campaign among Democrats in general, but it would help Dean's campaign among African-American Dems as well. Gore's support within the African-American community is probably only rivalled by Bill Clinton's (in fact, I think Gore received more votes from African-Americans in 2000 than Clinton ever received).
Gore supposedly announcing the endorsement in Harlem symbolically reinforces this, as well (Harlem is also where Gore "turned the tide" in 2000 in a debate with Bill Bradley).
This Gore endorsement, plus the Dean campaigns efforts in S. Carolina with Jesse Jackson Jr., plus the Dean campaign's efforts to get endorsements from CBC members, all point to a concerted and well-orchestrated effort to bring African-Americans on board.
And, of course, African-Americans are VITAL for a Democratic victory in 2004.