Mixing various styles, colors, textures, patterns, and materials creates visual interest and is a vital part of creating rooms with character. The opposite starts happening when you keep all those factors similar; you get a less lively, bland, and less satisfying look.
There are other benefits (that sometimes get overlooked in decorating) to this approach as well. It accommodates life and self expression better. Be it artwork, furniture, or something else, most of us like (and may acquire) a variety of items over the years and they would more easily fit in settings like these and even add charm. Also, things get broken, or worn out over time. It is easier to find suitable replacements and you will have more options to choose from when you already have a variety of styles, a more honest to life decor. A restricted color palette/theme/style will not accommodate those situations as well.
Sources: image 1, image 2, image 3, image 4