What was the advantage of the extra wheels?
Extra mechanical grip under acceleration and during cornering, as well as better laminar airflow over the front of the car.
You have to remember that at this stage (1976) wings had only been on racing cars for eight years, and constructors were still playing around with ideas and concepts that would be laughable today. Of course it was still very much pen-and-paper work then, even wind tunnels were still not really used for such for a few more years yet, so it was more often than not a case of "build it and let's see..."
Though the gain from running 6 wheels to get extra grip was marginal, and the concept was quickly rendered obsolete by Colin Chapman's introduction of side skirts. These created a much bigger pressure differential between the underside of the car and the bodywork and wings, effectively sucking the car down onto the track. The effect increased with speed, so the faster you went, the more the tyres were pressed into the track surface, meaning that the cornering speeds were far higher.
However, the downside of the side skirts was if the venturi under the car was disrupted, the car would fly up into the air.
