Historically, the Welsh had a patronymic naming system a bit like the Scandinavians. In the same way that Scandinavians suffix 'son' to a name, the Welsh prefixed ab or ap (meaning son of) to a name. In Scandinavia, Richard's son becomes Richardson. In Wales it would be Ap Richard... or Prichard.
As fixed surnames became more mainstream, the Welsh were left with a relatively small pool of surnames, which must have led to some confusion. This is probably the reason that surnames were combined with other identifying facts about an individual: their mother's name, where they live, or their job.
Those of my era will remember 'Jones the Steam', and the Newsagent in Holywell was called 'Morris the Observer'. In our family one of the maternal names stuck.