Question

why are poles of field magnet of moving coil galvanometer made concave in shape?

Answer 1

This helps produce an approximately radially symmetric magnetic field. The field lines are like radii of a circle spaced at equal angles.

This means the field strength is constant for different coil angles and the scale can be kept linear (doubling current doubles angle of rotation).

If the field strength changes for different coil angles, then doubling current would not double the angle of deflection.
Hope you can make oit my words..✌✌