The more candidates in the race, the better for Donald J. Trump, supporters say. And in a tight presidential contest, a small share of voters could change the result.

This works because almost all the US uses first-past-the-post elections for the Presidential general election. So you get outcomes like this:

Scenario 1:

Biden: 10 votes

Trump: 9 votes

Kennedy/Stein/West: 0 votes

Biden wins the state

Scenario 2:

Biden: 9 votes

Trump: 9 votes

Kennedy/Stein/West: 1 vote

Tied vote, decided by game of chance/lawsuit

Scenario 3:

Biden: 8 votes

Trump: 9 votes

Kennedy/Stein/West: 2 votes

Trump wins the state

This is why you see huge financial support from Republican billionaires for third party candidates who have no chance of winning.