Why use a 1-to-1 relationship in database design?

Question

I am having a hard time trying to figure out when to use a 1-to-1 relationship in db design or if it is ever necessary.

If you can select only the columns you need i

Answer 1

Just a few samples from past projects:

• a TestRequests table can have only one matching Report. But depending on the nature of the Request, the fields in the Report may be totally different.
• in a banking project, an Entities table hold various kind of entities: Funds, RealEstateProperties, Companies. Most of those Entities have similar properties, but Funds require about 120 extra fields, while they represent only 5% of the records.