They are an absolutely awful thing for the Left. They manage to pull off a few of the most left-wing people into their cause - which basically means that those people will now not contribute to anything that has a chance of success - and then they push the more moderate people out of the Left.

I do not believe most of the conspiracy videos/theories about them, but I can see why people would believe that Antifa was created to subvert and destroy the (American) Left. The Democratic Party is not going anywhere, but this definitely weakens them by cutting off both their left and centrist flanks at once. Honestly, if this were a conspiracy by right-wingers to create a movement to kneecap the Left, it would be a brilliant one.

We really do need multiple parties in this country.