Draw a labelled diagram of a moving coil galvanometer and explain its working. What is the function of radial magnetic field inside the coil?

The reason is magnetic flux is maximum through coil only if the face of coil is normal to the flux meaning the area vector of the face of coil must be parallel to the flux always , which you can understand from the following equation for torque i.e,

$T=BINA\cos \left(a\right)$ where a is the angle between area vector and magnetic flux , so you must have noticed that the T will be maximum only if $\cos \left(a\right)$ is $1$ i.e, parallel to flux