A two-note shell is the absolute minimum voicing: just the root and one other note that defines the chord quality. One variant pairs the root with the 3rd (telling you major vs minor), the other pairs the root with the 7th (telling you dominant vs major 7th). Two fingers, instant harmonic clarity.
Variant 1: Root + 3rd (or b3rd for minor, 4th for sus). Variant 2: Root + 7th (or 6th for sixth chords).
When you are a complete beginner and three-note shells feel overwhelming. When accompanying a singer very quietly. When sight-reading a chart for the first time and you need the simplest possible harmonic sketch. Graduate to full shells once these are comfortable.