From the Merriam-Webster Dictionary

Convex curve given to a column, spire, or similar upright member to avoid the optical illusion of hollowness or weakness that would arise from normal tapering. Exaggerated in Greek work of the Doric order, it grew more and more subtle in the 5th–4th century BC. Entasis is also occasionally found in Gothic spires and in smaller Romanesque columns.

Without adding the extra width to a column, they may appear skinny in the center. This happens more with taller columns. Thin columns do not look sturdy and able to hold up the weight above.