In a moving coil galvanometer, a radial magnetic field is obtained with concave magnetic poles, to have

Torque on the coil will be maximum at every instant when the angle between the magnetic moment of the coil and the magnetic field is 90⁰. 

So, in a moving coil galvanometer, a radial magnetic field is obtained with concave magnetic poles to have the plane of coil is parallel to the field in any orientation of coil.