Your WordPress installation contains a database for posts, pages, menus links, etc in a textual and encrypted data. Other things like the files you store in a file sharing site, videos, music, and other files aren’t stored in the database but in wp_content folder instead. As your file grows, your WordPress database also grows; thus, generating garbage data which make your site load slower.
Optimizing your WordPress database is an important task to keep your WordPress site in its best shape. An optimized database performs faster, and responses faster than a site that haven’t had the site optimized in years.
Read the tips on how to optimize a WordPress database in this blog: http://www.wpexplorer.com/optimize-wordpress-database/