Mobile Cache used for caching mobile devices/browsers and it's enabled by default.
If you want to use different cache file mobile, you can enable "Use separate cache file for mobile." If you are using different theme for mobile (ex: WPTouch) you might want to enable this. You don't have to use this option for the responsive themes.
For the developers:
When you need to support different devices, take a look powered_cache_mobile_browsers and powered_cache_mobile_prefixes filters.