We run an experiment, beam results to a friend on a space-ship,
and they use them.
But the numbers they'd get differ ... for all this to work they have to
‘translate’ ours.
To do physics we have to agree on using Special Relativity;
it is one of the basic laws.
( But: it applies to intertial frames only – no gravity;
and ... spacetime is simply there. )
More than relating intervals for moving friends, it goes deep into the most basics notions.
Space and time are not well-defined concepts on their own; the construct of
“spacetime” is.
But then remember, this also means that mass relates to energy; for example :
E = mc2.