Ultimately, It is always about GOTV. Historically, under 60% of registered voters actually vote. With enough ground game calling up registered voters, driving them to the polls, etc, that can make or brake a campaign.
Think back to Romney’s (I know it pains me to bring up that albatross of a candidate) campaign. His campaign decided to use an automated computer system, code named “Orca” to GOTV. Orca died on the beach. It was a complete debacle of a deployment.
https://www.businessinsider.com/romney-project-orca-disaster-2012-11
They are assuming 2012 black numbers for Biden.