geometry.getArea() versus getArea from ol/sphere

Question

I am trying to understand the difference between:

1. polygon or multipolygon getArea (documentation)
2. getArea from ol/sphere (documentation)

The documentation of #1 reads:

Return the area of the polygon on projected plane.

The documentation of #2 reads:

Get the spherical area of a geometry. This is the area (in meters) assuming that polygon edges are segments of great circles on a sphere.

Can someone clarify this for me? I want to measure areas (application is agriculture / cadastre related) for EPSG:2100. If I understand correctly #1 should be used and not #2