Moose and friends inserts subs for its helpers (e.g. has) into the package in question. The no Moose incantation runs its unimport method which removes those subs from the package's symbol table which means that you can't accidentally or intentionally call those routines (an extreme example, but in another compilation unit you couldn't do { package Foo; has 'new_attrib' => ( ... ); } since Foo::has is no longer a valid sub). It's not "necessary", but it's "cleaner".