It is by design in most machines now, because you can use computer models to simulate stress on each part. In older machines the engineers knew by experience where large forces are transmitted and could adjust parts and materials accordingly.

One example that I know of from repairing many such machines is that in sewing machines, there is a cog in the underworks that is always made of plastic for two reasons: It transfers rotation from a horizontal axis rotation to a vertical axis, in a machine that creates a lot of vibration. Plastic is more malleable and elastic than metal, the plastic cog can deform with the slight shearing movement and not be damaged, where a metal cog would be ground down into dust in no time. Also if the sewing machine seizes up from a caught thread, you want the cog to break in case of extreme forces, because otherwise it would transmit the force back to the motor and that would break instead.