Why does this matter?  Because polls can help steer public sentiment toward one side:

The researchers asked a selected group of voters to state their opinions on a variety of real public policy questions, and then presented them with fabricated poll results on the same topics. When the test subjects learned that a large number of experts favored a position, opinions shifted by 11.3%. But the "opinions of people like me" changed opinions by just 6.2%, while a general poll saying that a majority of people favored one side or the other moved the needle by 8.1%.

https://www.gsb.stanford.edu/insights/how-polls-influence-behavior

So if positive polling for Biden leading up to the election moved the dial by even 5%, that could explain several of his purported victories in battleground states.

In addition, polls can help with fundraising, as people will be more likely to donate if they think their candidate has a good chance of winning.