What is the use of the inertia delivered in the characteristics
of a stepper motor?
Please post the link to that datasheet.
Usually the inertia of the entire build must be considered in order to set acceleration/deceleration and avoid stalling at start or run over when stopping.
Hi,
Inertia figures are given to help, if needed, calculate rotary forces needed to get the stepper up to and down from speed.
If you are using a stepper for position work and you have it moving at relatively high rotary speed, you may need to calculate the energy required to speed up and slow down the motor just on its own, before considering any load that it is driving.
A high inertia motor will be harder to speed up or force to slow down.
In most applications the inertia is not that important, however in the field that I work in, it is as the motors are required to stop, start, run, stop to very precise position and speeds.
(These steppers are used as servos and have >20kg mass.)
Most of the time in hobby applications, experimental trials are easier to do, rather than the maths.
Tom... 
Ahhh I understand, I was using it to calculate the acceleration ramp without losing steps. But there are some frequencies that behaved a bit strangely (300RPM), I assume it could be the resonance, but I don't know if it is directly related to the inertia of the motor.
this is the datasheet buddy
23HS2430.pdf (362,2 KB)
Inertia is not stated.
The section that says rotor torque is the inertia by its units of measurement by dimensional analysis I have MR^2. They just translated it wrong.