Putting a vehicle on the portal will add it to a digital collection. One isn't required on the portal when actually racing.
I believe there is a 2-player mode when racing.
As far as pure racing goes, there isn't as much of a benefit to the original...except for a few key factors. When doing the racing mode in Imaginators, I found that the weapons do far more damage than in Superchargers, likely because there are no armor or weapon upgrades in the former. You also can't mix and match different custom parts, and are instead stuck with the preconfigured variants. I'd honestly recommend playing the Superchargers racing mode over Imaginators for those reasons.