What does simple MODIFY do without changing the parameters in ALTER TABLE (SQL)?

Question

alter table emp modify mgr; 

Are previous restrictions discarded / retained?
What is the use of this?

Answer 1

From the documents

modify_col_properties

Use this section to change the properties of a column. Any of the optional parts of the column definition (data type, default, or restriction) that you omit from this sentence remains unchanged.

So this is no-op if you do not specify any properties. This in itself is not very useful.