Why do we place guards over capacitors and where does the guard voltage come into play when using an analogue L-C meter to probe the capacitance?

Why do we place guards over capacitors and where does the guard voltage come into play when using an analogue L-C meter to probe the capacitance?For instance, we have an isolated spherical capacitor on an insulating rod with a conducting base. Why do you get the most accurate vale for capacitance when applying a guard voltage to the base and then probing the sphere instead of grounding the base or doing neither?