Why does Newtonian mechanics include relative motion between inertial frames?

[am2010]

I know that in Newtonian mechanics an inertial frame of reference moves relative to absolute space. But why does Newtonian mechanics include the contention that two inertial reference frames move relative to each other?

It seems that if you have an absolute reference frame then there is no need for reference frames that can also move relative to one another.

 

[K^2]

There is no absolute frame in Newtonian Physics. There are infinitely many inertial frames. They are all good choices. Furthermore, Newtonian Physics does provide you with ways of working in non-inertial frames, but fictitious forces must then be included.