Not an american thing, but rather old-fashioned. A long time ago, well-to-do Britons had a "family lawyer". Not on retainer; but he knew the family business (so didn't need briefing), and he dealt with wills, property, that kind of thing. Like a family doctor, rather than a specialist consultant.
Nowadays only the very rich can claim to have a "family lawyer".
My middle class family certainly has a "family lawyer" who deals with wills, property, that kind of thing. You should use a lawyer for those things, why not use one you know and trust?
If we were very wealthy, perhaps we'd have a legal matter for him every month instead of every year or two, but that's plenty of time to build up a relationship. Similar to having a family doctor.
Nowadays only the very rich can claim to have a "family lawyer".