The alignment of text in a novel is fully justified, meaning the left and right margins are flush (even). The software achieves this by stretching out the spaces between words to a greater or lesser extent. Professional layout software also adjusts letter spacing (to a less obvious extent), which Word doesn't do.
The longer the words you use, and the larger the font (i.e. the shorter the line length), the more likely it is you'll get ugly spaces between words. In the sample below (I've increased font size to emphasize the problem), the first paragraph looks fine, but the words in the second paragraph are widely spaced because the word "important" didn't fit on the previous line: