> Immigration authorities say the move is aimed at preventing cases in which foreign workers obtain visas under one category, but then engage in unrelated or lower-skilled work.
The claim appears to be that people were using up visa slots for things like interpreters or other jobs where clearly you'd need good language skills to actually do the job, including in Japanese, with the intent all along of doing some other job instead. An up-front test should let through almost all of the legitimate claimants of these visas, and stop almost all the fraudsters. Probably a lot cheaper than a similarly-effective level of after-the-fact auditing, or more-extensive checks into applicants' work situation.
[EDIT] I mean, in the framing provided by the government, the above appears to be what's going on. Governments may lie, of course.