The thing is, if you have a method like:

sub get_name {
   my $self = shift;
   return "Bob";
}
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... it's true that this implementation of get_name doesn't make use of $self to determine the return value. However, other implementations of get_name (perhaps in child classes, parent classes, or even sibling classes; perhaps in classes that have not been written yet, but might in the future) might need to refer to $self to return a value. Making get_name a method call rather than a function call is beneficial for polymorphism.