!default Responsive variables

[willit0]

I was trying to define custom values for the $breakpoints variable but I wasn't able. Just found out there is no !default at the end of those variables. Any specific reason for that?

$breakpoints: (
'palm' '(max-width: 480px)',
'lap' '(min-width: 481px) and (max-width: 1023px)',
'portable' '(max-width: 1023px)',
'desk' '(min-width: 1024px)'
)!default;

$breakpoint-has-widths: ('palm', 'lap', 'portable', 'desk')!default;
$breakpoint-has-push: ('palm', 'lap', 'portable', 'desk')!default;
$breakpoint-has-pull: ('palm', 'lap', 'portable', 'desk')!default;