They enable full, open counters and unmodified character shapes. Many bold and black designs become only caricatures of the original designs, with very small counters and drastically modified letter shapes. In serif typefaces, individual letter legibility begins to suffer as serifs take on exaggerated shapes Very long serifs, or those that are exceptionally heavy, detract from individual letter legibility, as do those with unusual or highly modified shapes. Ideal serifs are somewhat short, slightly bracketed, and heavy enough to be obvious but not very obtrusive. ITC Novarese and Plantin have perfect serifs.
Graphic designers have no control over typeface legibility.
Typeface designers do have that control. While not all typefaces should be designed to be paragons of legibility, those that are intended to be used for text or lengthy display composition should be drawn with the above guidelines in mind, otherwise you are not creating tools that will give the expected results.