Given the context, I presume the question was introduced in an attempt to give purchasers reassurance that any electrical work had been undertaken competently, in compliance with relevant regulations and with all the necessary 'approvals', where applicable, having been obtained. Don't forget that, long before 2005, there were similar questions there in relation to all sorts of other building etc. work.Surely the question was introduced merely to justify and validate the notification and self-certification (self-notification) procedure and Schemes.
I suppose it is in some ways comparable with car purchase, a buyer will often ask for a 'service history' - but, just as with houses, such is very often 'not available'. Indeed, as with house electrics, all of the car's servicing may have been 'DIY'.
I imagine that the thinking behind the lawyers' attempts to get answers to these questions stems from a fear that (as could be the case with, say, other building works and planning etc. matters) the buyer could face some sort of 'come back' if they bought a house which had undergone notifiable electrical work without it having been notified. However, as we know, it would seem that, in practice, this never actually happens.
Kind Regards, John