To obtain a more pleasant effect
Width Proportions of Modern Roman Capitals [Fig. 3] (Source: Brown, Frank Chouteau: “Letters & Lettering: A Treatise With 200 Examples,” 1921)
Typography is the art and technique of type design. Each letter and character in a font is drawn in a very precise way and the designer thinks about how the letters will interact with each other. The outlines -- the shapes, curves, and dimensions -- of each character make them come to life. Look at the tail of the three "R" letters in the figure -- each gives a different feeling. No matter what the words say, the letters themselves are beautiful and convey something on their own.
“Width proportions, which may be found useful in laying out lettering for lines of a given length, are shown in [Fig. 3] in a more modern style of the Roman capital. In the classic Roman letter the cross-bar is usually in the exact center of the letter height, but in 3 the center line has been used as the bottom of the cross-bar in b, e, h, p, and r, and as the top of the cross-bar in a; and in letters like k, y and x the “waist lines,” as the meeting-points of the sloping lines are sometimes called, have been slightly raised to obtain a more pleasant effect.” (p. 6) -- Letters & Lettering, 1921