It is a well known fact that WordPress is a very flexible CMS. In fact, many theme makers nowadays rely on WP’s flexibility to add additional functionality to their themes. For instance, certain themes come with SEO settings, shortcodes, custom post types, and of course, sliders!
While having such added features in a theme is awesome, at the end of the day, if you really wish to retain such functionality on your website even after you change your theme, you should consider opting for a plugin. Furthermore, what if the default single slider on the homepage of your theme does not really work for you? In this case as well, a plugin can help you out.