I mean, I don't think they're taking about a full diagnostic. Just the code associated with a CEL.
It'd be nice if you could read the code from the dashboard or infotainment without digging out a code reader.
And it'd be even better if they had human readable descriptions for those codes, especially for OEM specific codes.
For most people, a CEL is all you really need. But sometimes and for some people, just telling them the problem would be super helpful.
For example, a loose gas cap is a CEL. Save people $100 at the mechanic if it was just like "check that your gas cap is tight"
It's almost always the reverse.
Regulation is what forced manufacturers to provide standard error codes over a standard protocol using a standard socket, so that people could self diagnose their car problems without getting locked into their dealership mechanic