From the php manual regarding "password_needs_rehash"
"password_needs_rehash — Checks if the given hash matches the given options"
Which means it is needed if the option parameter is changed.
Option were not given by the user the manual says
"If omitted, a random salt will be created and the default cost will be used.
The question here is
If I do not change the default cost, why would I need to use the password_needs_rehash?