Beam polarisation is an integral part of the physics case of future Linear Colliders. In this contribution, important examples from Higgs coupling measurements, top and electroweak physics at high energies, the $Z$ pole program as well as from searches for production of new particles will be reviewed. The full exploitation of its advantages requires the polarisation to be known at the permille-level. The polarimetry concept which has been developed for the ILC to achieve the required precision based on the combination of Compton polarimeters, spin-tracking simulations and a global analysis of collision data will be presented.