Polls are different from elections, though the difference can feel subtle. It's a mistake to assume that because pollsters project that an election is a foregone conclusion, that it won't matter whether you vote. Unfortunately, predictions are often wrong. Because people with xenophobic and racist tendencies know that their views are socially unacceptable, they're less likely to report their true feeling, even when they're polled anonymously.
While the Bradley effect is disputed, the fact, that the Brexit result was a surprise, reinforces how infrequently I interact with people who hold drastically different political views. Based on my social media feeds, I assumed that the referendum would've been a landslide, but it ended up being close and decided in the opposite direction.
For the democratic process to work, people need to vote their conscience even if it feels like a formality.