I think the idea is that the concept of 'a name' is actually a bit more difficult than programmers often assume. Some cultures only have a single name, some have a 'private' name that, say, the government would use, and a different name which is used everywhere else. And a whole host of other differences.
So imagine someone from a culture where a single name is ok tries to sign up to your application but you insist that they have 'forgotten' to fill in their second name. That might make them a bit annoyed. I used to work with someone who actually quit his job because the organisation insisted on using his 'private' family name everywhere rather than what he looked on as his 'actual' name which was a very, very big deal to him - and I can understand.
Have a read of https://www.kalzumeus.com/2010/06/17/falsehoods-programmers-believe-about-names/ for some more examples. I think the current 'best option' is to ask for a 'formal' name if it's needed and a 'what would you like to be called?' option - but it's a bit of a minefield.