The mobile site is simply a copy of your website in which the server delivers an optimized page for easy navigation. It will offer you simpler navigation and faster load times.
Mobile sites are built to fit a mobile browser, so you don't have to make your user squint and stretch their mobile screens. With a mobile site, the site building is just one time.
You do not need to build it separately for the different types of mobile brands. It is just a onetime installation. Users don't have to remember the site name. Just a click on the icon will take them to your website.