He's using the by-election to pause the investigation into his finances, and hoping to use an election win as a mandate and proof that he did nothing wrong, because a crook could never win a fair election, right?!

The only way the other parties can respond to this, which is what they're all now doing, is to not stand a candidate, thereby no vote is cast that can be used as Farage's mandate/retribution, and the investigation will continue, which will hopefully lead to him being removed from his seat, and a proper by-election will then be run.

Basically, Farage is once again trying to distract from his wonton corruption by playing political theatre, distracting the masses, and giving himself a way to save face, or even turn it around to a win.