Think about it: if you support Trump, you're "red-pilled" to at least some extent, and so seeing Trump low in the polls isn't gonna change your opinion.
The true purpose of these polls is to RETAIN voters for the left. They should be called "retention polls".
There are likely many people who have bought into leftist ideas at some point who are now questioning these ideas, after being exposed to good counter-arguments and data. They may also have people they care about who strongly support Trump / oppose the left
That's where "retention polls" come in - they see that "most people oppose Trump and agree with the left", so they stop questioning and go back to the leftist bubble.
They want to demoralize weak Trump supporters and fence-sitters. Also, they are trying to pump up their base. Lastly, they are setting the stage to claim, Trump had to cheat in order to overcome the huge deficit in the polls.
I don't think there's such a thing anymore as a "weak Trump supporter". If you support him, you don't disagree with what he always says about the dishonest, Marxist Democrats and their media. If you believe that and you aren't a Marxist, you're not a weak supporter and won't be swayed not to vote for him just because he appears to be less popular. If you ARE a Marxist, you're not a Trump supporter.
I think the polls are primarily to retain questioning Democrats, but also to gain fence-sitters (as few as there are), and to make the fraudulent voting they're planning seem more believable.