When we need to use 1-to-1 relationship in database design?

Question

When do we need to use a 1-to-1 relationship in database design? In my opinion, if two tables are in a 1-to-1 relationship, they can be combined into one table. Is this true

Answer 1

yes in general.

One exception may be if you want to assign privileges differently to a subset of columns.

also consider that this is true only when both sides are required.