There are a number of people on the right who won't support a racist, but are still stupid / naive / gullible enough to believe his current lies. 'Farage doesn't hate black and brown people, he's just protecting us from the nasty refugees', those kind of morons.

If there's evidence that he's always been racist, some of them might stop supporting him and vote for someone else, lessening his chance of getting any real power.

It's a slim chance, but it's worth pushing.