since the car has a "skirt" that comes very close to the ground my guess is that the fans is used to pump air out from under the car, to create an extreme downforce ... just like a hoovercraft .. but working the opposite way.
Like the airducts that widens at the rear on todays cars, it has the same effect.
There was a F1 car (Alfo Romeo ??) that had a similar huge fan at the back too ... and it had a huge downforce with it going. the FIA was quick to ban fans like that though
The 2J was a "sucker car". Before Lotus built its 78/79 F1 cars, Jim Hall, and just maybe Chevy's unofficial race shop (some folk say Chevy built this) developed ground effects by running those fans with a snowmobile engine to pull air out from under the car. Officials banned the car after it had run strongly but unreliably in a couple of races because (a) it contravened the FIA's ruling that cars shouldn't have movable aerodynamic surfaces (wings had fallen off struts on F1 cars with exciting results, but of course Chapparal and Brabham, with their BT46B, which actually won the only race it competed in, claimed that the fans were primarily for cooling, not aerodynamics, and thus legal) and (b) drivers following claimed the vacuum effect sucked the grit off of the racetrack and sprayed them with it! Motor Trend carried an article that claimed the car was developed by Chevy engineers and was given to Jim Hall after they turned down a proposition from Roger Penske which was that Penske develop the car in secret until it was capable of breaking the track record at Riverside by ten seconds, that it be entered and perform to that level in practice, then be withdrawn with the disclaimer "Chevy isn't in racing, but if we were, this is what we'd run!". I think the Chevy powers-that-be just wanted to see it racing, so Penske was refused.