My friend explain, what happen:

If I have function, which makes Inserts, Updates or Deletes on MyTable, I cannot call this function with 'select package.MyFunction() from dual', because, same call can be 'select package.MyFunction() from MyTable' too, and Oracle doesn't know (if function modified MyTable), what return.

It works, if is function calling from PL/SQL:
declare
promX NUMBER;
begin
promX := package.MyFunction();
end;
/

Pavel