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

One reason would be to put frequently accessed data in one table and extremely rarely accessed data in another table. It would run faster and save some memory.

But I would have to have my arm twisted hard before I would do it.