Not the Democratic Presidential nominee but the Democratic Party.
Basically, a Nader run will allow a second anti-war choice if a pro War Democrat is the nominee and that would likely keep the Greens under 5% nationally, denying them future federal funding.
There will definitely be an antiWar candidate even if Democrats don't nominate one but splitting the AntiWar vote would at least remove that federal funds angle that would threaten Democrats in future presidential elections.