It is relevant to reduce the quantum noise in the system. If the shield strength is more than 50,000x, better the quantum computing system is shielded from the earth's magnetic field and thus better quantum noise reduction.
, at least, theoretically.
Superposition is the heart of quantum computing. Superposition state is susceptible to fluctuating external magnetic fields, thermal fluctuations, radiowave etc., The quantum processor should be in an space where the magnetic field is uniform and stable to avoid quantum noise introduced by the above mentioned factors. Thus, isolating the quantum computing system from its disturbing environment is mandatory.
Achieving a ideal quantum noise free environment is still a daunting task. However, progress made thus far has brought us to experimental realizations of quantum computers. Shielding more than 50,000x earth's magnetic field would reduce the quantum noise induced by earth's magnetic field.