For some time now the rally field has been classed as a public right of way and subject to the same rules and regulations as any normal road, hence the need for insurance to cover road risks.
Whilst the club insurance does cover you to the value of £5,000,000 for public liability if you exhibit a pedestrian-operated machine, it does not extend to machinery that is classed as “ride-on” because of the huge variety of machinery that it encompasses. That’s why “ride-on” machinery has to be insured separately.
I know pedestrian-controlled machinery has an equally wide variety but, because they are pedestrian-controlled they tend to operate at a much slower pace and so their capacity to do damage to the general public is deemed to be a much lesser risk.
I hope that explains why rally organisers insist on insurance.