my understanding is at the turn of the century, it was undignified for a lady to ride on the pillion. Hence, if a gentleman wished for his mate to also enjoy the pleasures of motor biking, some sort of attached chair was necessary. The forecar was one option, albeit a bad one, as was a towed car, which failed because of exhaust and dust. Hence the sidecar, even with it's inherent faults, was the best solution.