Spherecast Arguments

What’s the difference between the SphereCast radius and maxDistance arguments?

SphereCast is the radius of the sphere (basically, SphereCast casts a thicker ray, so the radius is the thickness of the ray).

maxDistance means how much long ray will detect collider.
The main difference why we use SphereCast instead of raycast is the thickness of the ray.
SphereCast is useful when a Raycast does not give enough precision, because you want to find out if an object of a specific size, such as a character, will be able to move somewhere without colliding with anything on the way. Think of the sphere cast like a thick raycast. In this case the ray is specified by a start vector and a direction.