Greek and Roman columns would have a slight curve because it was more pleasing to the human eye: https://en.wikipedia.org/wiki/Entasis

> Its best-known use is in certain orders of Classical columns that diminish in a very gentle curve, rather than in a straight line as they narrow going upward. The human eye would allegedly perceive that the middle of the column was diminishing in a concave curve halfway up the column, and entasis corrects this.