Hello Eitan, to clarify.
When you add your profile to the Singers and to the Songwriters, nothing more happens.
On the other way: When you add your profile to Singers-Songwriters, then nothing more happens directly. but implicit you say: It belongs also indirectly to Singers and to Songwriters.
In WikiTree you should avoid unnecessary categorization.
As Jim and Natalie mention, some categories are for famous people (with Wikipedia articles in various languages or similar).
For famous people (notables):
Canada, Musicians; Ontario, Musicians