Can someone explain why it contributes to his re-election?
Because it raised his popularity as measured by his approval rating. This has been said multiple times.
True, his approval rating has gone up slightly in some polling. And, Trump has supposedly used the impeachment proceedings as a way to generate additional campaign contributions.
https://time.com/5744415/trump-campaign-impeachment-facebook-advantage/
https://www.politico.com/news/2020/02/05/impeachment-blockbuster-fundraising-lawmakers-110770
However, that doesn't necessarily mean the impeachment was a mistake, for a couple of reasons:
- It may have caused Trump supporters to rally around the racist, but it also may help energize Democrats as well. (Many democrats also saw an increase in fundraising during the impeachment proceedings)
- Its hard to know how the alternative might have gone. Ok, so Trump's approval ratings ticked up by a little bit. If they didn't impeach, it may still have gone up. Or it may have stayed the same.
- It may have a secondary benefit on some of the congressional races. Senators like Collins (who was already dealing with low approval ratings) now has the stigma of her "Trump learned a lesson. He didn't? My Mistake". This may play a part in her 2020 senate race.
Overall, I don't think we can say whether there was any net benefit or harm, and we probably won't for some time. Trump's approval ratings increased in some polls, but there is no guarantee that will be sustained. And it should be noted that many polls showing increase in his approval ratings are national, which doesn't mean he benefited at all in battleground states like Florida or Wisc. (For example, his approval rating in Michigan was 42% last summer. After impeachment, his approval is... 42%.) And Trump may have 'survived' impeachment with his support base intact, but between Trump's questionable actions and the possibility of future revelations, he may still get damaged.
https://morningconsult.com/tracking-trump-2/