Records attach to the property. Flats change hands and landlords change agents, but the installation stays, and the firm holding the history of that consumer unit is the one called back.
Renewal dates are future revenue already earned. Every report has a date on which it expires, and a reminder a month before produces a booking with no marketing whatsoever.
Test results and photographs are evidence. Recorded on site and attached to the job, they answer a query a year later; left on a phone, they are gone with the phone.
Materials disappear into the van. Cable, accessories and parts bought for one job and used on another leave both mis-costed, and twelve months later there is no way to tell which type of work carried the business.