或: how did "region" become "either"?
without implying that the classical explanation is necessarily correct, worth noting that 康熙字典 thinks 或 was the original single character for 惑, before the latter character was introduced. Not hard to see how the grammatical usages of 或 could have evolved while it still also meant 惑. Or the other way around, the grammatical function could have been original, from which the meaning of 惑 is easily derived.