At first, I thought this was sufficiently similar to no.3 (“become acquainted with every art”) as not to need a separate post. But not.
Certainly, just as the good samurai should have a proper appreciation of all the arts, so as to make good judgements on behalf of his lord, his daimyo, then the same applied to the professions (from builders and other craftsmen to medicine and law). As the manager of his domain, this was required. But for the professions, this went further.
Knowing the Ways of the professions meant having a deep understanding of what it was those professions had to do to deliver their goods and services. There was no value in trying to cut corners, because the Ways of professions were part of the warp and weft of life.
So it is now. If we are testing an application which addresses a particular function in law, or education, or medicine, then it is no good imagining that we can test independently of the special requirements of that profession. Business knowledge is invaluable in understanding how users will react with an app, what they want from it and how they will use it. If we are lucky, these things are in the spec or the requirements. But sometimes they are not. Or we may not have access to the original specification; or, indeed, to end users themselves.
For that reason, there is a good case for companies to ensure that a testing team has at least one person who has business knowledge; preferably of the business specialism itself, if the app is so specialist, but otherwise any sort of wider business knowledge will be useful.
If the company only has one tester, then business knowledge is essential. Indeed, it may be preferable to technical knowledge. Technical knowledge can be taught; experience cannot.
But if this is not possible, then at the very least it will help if the tester can visualise the end user and the way they will probably want to interact with the app. Any sort of life experience may be turned to this end.