If the text is too small, readers may have trouble interpreting the text. While choosing a point size may seem trivial, it can impact legibility. Consider the difference in the example below. There are several point size of the Times New Roman typeface. 
If black text is set against a dark background it may be difficult to read for some readers. Consider the difference in the example below. Black text on a white or light-colored background is easy to read, while black text on the blue or dark-colored background is hard to read. 