Buffering: Planar vs Geodesic measurement mode when data is in GCS

[Craig_Eissler_Iceman]

See excerpt below from the Buffer Help doc.
I'm not certain I completely understand the second bullet.

...Why would this option even be allowed with data in GCS?
...Perhaps I'm not clear on how a distance input in degrees even works..?

However, I did notice that when inputting linear units I get the same result as the data in PCS but using geodesic mode.

[DanPatterson]

angular distance is a real thing, but it doesn't mean that the planar distance will be the same at all locations along a path.  Picture a line of longitude... buffer it by 1 degree... you end up with a lune.  Geodesic buffers should be used.  Your "however" shows this

[Craig_Eissler_Iceman]

But, if my data is in a GCS, how is it able to use linear units to measure the buffer distance?

For instance -- see below: If my Map is in a GCS and I attempt to Measure in Planar mode, it is grayed-out.
So, is this not the equivalent concept?

[DanPatterson]

No