I think there's two main reasons this keeps coming up every time a poll shows that Harris has even a chance at winning:
In 2016, Hillary Clinton was way ahead in the polls, only to lose. This was due to many factors, but one that gets a fair amount of blame is that a lot of people hated her and didn't want to hold their nose and vote for her. Staying home was a lot easier when she was supposed to win by a wide margin.
While a lot of people pushing third party candidates were never going to support Harris, there are also those who think that it's ok to vote third party in states that aren't seen as in play in order to "send a message" without risking a Trump win. But if the polls are off and that state is closer than expected, those votes could still cost the election.
Historically, being ahead in the polls helps a candidate. The bandwagon effect is real, and can help drive up turnout and shift how people perceive an election or issue. That's why partisan polls designed to skew the numbers have been around for so long.
Either way, it doesn't hurt to remind people that no matter what they expect the outcome to be, we need as many votes as we can get. Bigger margins can help fight off conspiracy theories and legal challenges, and more turnout in down-ballot races can make the difference between gridlock and real progress.
Some think that if a candidate is doing well or poor enough they won't bother to vote because it won't matter. Kind of like when everyone assumes someone else called 911 when there's an emergency.
Do we have any evidence that that has ever happened, ever ever? Based on the exit polls, we didn't see that in 2016 (Democratic turnout was about what we expected it to be). And we know that people really like to play for the winning team, even if the team is already winning.
I am pretty confident I've heard that the opposite is true--that hopeless feeling like the other side is certain to win, and your vote will not change anything, can get people to stay home--but I haven't heard anything compelling suggesting that complacency can get people to sit out an election.