You don't have to join the electoral roll to be an active part of St Andrews but joining the Roll is compulsory if you wish to:
Stand for election to the PCC or other synodical council.
Attend the APCM (our annual church meeting) on May 11th and participate in voting.
The other big benefit to St Andrews is that a stronger, more active electoral roll can lead to better representation for our church in the wider Church of England, like at deanery synods, or diocesan synods.