Sometimes when theme developers build their themes they incorporate a plugin in or two. The most often included plugins are sliders. The issues is that if a plugin is bundled within the code of the theme, there is no way for it to be updated automatically. It will become an issue as soon as the next version of the plugin is released.
Be careful about the themes you choose to use. Ask the theme developer about what plugins they've bundled into their themes. Better yet, ask them if they use TGM Plugin Activation which allows them to indicate to the user the plugins their theme needs but doesn't actually include them. Instead it includes a message in the dashboard that tells the user what plugins they need and offers to install them.