If your primary focus as an electrician, plumber or other tradesperson is SME or industry, of course, then there is far less customers.
At least here in Germany, in most rental contracts the responsibilities are split between landlord and renter, with the landlord being responsible for the heating, the water mains and the large fusebox, but the renter has to deal with everything under the scope of "Kleinreparatur". Exchanging faucets and siphons, wall sockets, installing laundry machines and other appliances, installing lamps, that kind of stuff many people (like me) can DIY but other people (like a lot of my friends) can't or don't want to, and they call in the professionals for an hour or two. And these guys have like 4-6 service calls a day.
Yeah and that plumber has 1000s of customer names, addresses, phone numbers, mail addresses, in Europe maybe even SEPA Direct Debit data.
Not funny if that kind of stuff leaks, especially not if you're, say, a person with a public register block due to domestic violence or whatever (basically, one step before witness protection that gives you a whole new identity).