It is still a mystery why only a small fraction of quasars contain relativistic jets. A strong magnetic field is a necessary ingredient for jet formation. Gas falls from the Bondi radius RB nearly freely to the circularization radius Rc, and a thin accretion disk is formed within Rc We suggest that the external weak magnetic field threading interstellar medium is substantially enhanced in this region, and the magnetic field at Rc can be sufficiently strong to drive outflows from the disk if the angular velocity of the gas is low at RB. In this case, the magnetic field is efficiently dragged in the disk, because most angular momentum of the disk is removed by the outflows that lead to a significantly high radial velocity. The strong magnetic field formed in this way may accelerate jets in the region near the black hole, either by the Blandford-Payne or/and Blandford-Znajek mechanisms. If the angular velocity of the circumnuclear gas is low, the field advection in the thin disk is inefficient, and it will appear as a radio-quiet (RQ) quasar.