John Edwards was added to the ticket
because he meets the prime requirement of any vice-presidential candidate - " First, do no harm ". He brings none of the inevitable difficulties or ego of a Hillary Clinton or a John McCain nor is Edwards a tired and colorless retread
In addition, Edwards, despite his slick trial lawyer background has never detached from his southern roots and he can still " speak Bubba " to a region that views the Democratic nominee with suspicion. Given that Kerry is the epitome of a stereotypical limosuine liberal and cold and arrogant Yankee moralist, Edwards bring much needed charm and warmth to the Democratic ticket. In practical terms, Edwards can bring little else to the table having had virtually no impact on any substantive national issue whatsoever - something of an advantage because it leaves very little record - barring unrevealed skeletons - for GOP operatives to attack except youth and inexperience ( which will stick because Edwards looks like he might be about 30). I don't see Edwards having much of an effect either way on the election unless he makes an unexpected gaffe.
Not a bold choice but from Kerry's perspective the most attractive safe choice.