Why would a database have a single field in a separate table? -

i have database large-ish primary table has several attached secondary tables. each secondary contains 2 fields: foreign key attaching principal table, , single piece of data. this:

this looks me it's semantically equivalent having nullable field on principal table, i'm wondering:

1. is there advantage having nullable fields split out own table?

2. if there's no advantage, there historical reason schema might have ended way?