When setting your type flush left / ragged right, the wordspacing and letterspacing remain consistent. You will not find “rivers” in blocks of text set this way. Some people perceive ragged right as less formal.
With justified text, wordspacing and letterspacing vary within the tolerances you set. Justified text sets more tightly (you get more words per page), but you get more hyphenation and, sometimes, unsightly loose or overly tight lines. Editing and manual hyphenation can usually rectify these problems.