Generally Planes are useful for generating Sketches or End Conditions but if there is not already a Default Workplane or a Flat Face, then you will need to generate your own Plane
Planes require References. If you already have items selected, such as two faces, a face and a vertex etc. the Reference Fields (BlueBox) will automatically populate.
With a single face/plane selected as your reference, the default option is an Offset
Knowing this, typing in a value and Check-It-Out will quickly generate an offset Plane
In most cases, there was a better solution to generating an Offset Plane
With a two faces/planes selected as your reference, the default option is an MidPlane
Knowing this, instantly Check-It-Out will quickly generate a symmetric plane to mirror about
If you're careful, you can use MidPlane Extrusion and strategically place your origin such that you won't need this step