That would work if you trimmed the sphere with a line, since the line will project to a planar cut in the sphere.

But if you draw a wiggly curve as the cutting object, the resulting cut will not be a flat planar curve, that's when you'll have to do those different steps.

Quick note - Planar also works on surfaces/solids too, in addition to curves. If there are any open edges of a solid that form a planar loop, it will create a surface there and join it in automatically.

For example - draw a circle, extrude it with cap turned off - now you can select the extrusion and hit "Planar" to seal off the caps.

- Michael