Why do database designers deliberately create one-to-one relationships?

e.g. You can have a table with 50 columns, or you can have two tables with a one to one relationship, having 25 columns in one and 25 columns in the next (the foreign key acting as a primary key). Maybe the first column is called "Contact_Details" and the next table "Contact_Details_Extended"

What's the big advantage? Something tells me Frans Bouma might have an opinion on this. Anyone? Bueller? Bueller?