This event was an important opportunity for Webb to address a crowd of kindred spirits. On the campaign trail he often highlights his working class upbringing and flashes his union membership card. Union members could be a key constituency for his campaign (well, as long they don't endorse someone else), and might find Webb more relate-able to them on a personal level than some of the other candidates.
"Organized labor is not the enemy, organized labor is your friend," Webb repeated several times in his turn at the podium, arguing Democrats need to do a better job of explaining the benefits of collective bargaining to voters.
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Jim Webb, Webb Campaign Press Release - Jim Webb Shines At Labor Convention Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/313244