Notice that the space between words is uniform in left-justified text.

In justified text, the spacing between words is irregular, slowing down the reader. Because a big space suggests a break between sentences, not a break between words, readers can become confused, frustrated, and fatigued.

Notice that the irregular spacing not only slows down reading but also can create “rivers” of white space. Readers are tempted to concentrate on the rivers running south rather than on the information itself.