Quickly turn your site on or off for your visitors, via a nice and easy dashboard.
You shouldn't really need to do it - but there are times when you might need to turn your site on or off for visitors. it could be some form of massive update or site launch and you don't want the site accedentily getting out.
Maintance Mode provides a dashboard in the settings section where you can flick your site in and out of maintance mode.
You can also choose to restrict the site to people who are logged into the back office of umbraco. So while everyone else sees your holding page, you and your editors can check the site out and make sure everything looks OK.
Usage Notes (Check these first):
Maintenance Mode will stop your pages from being seen, but any documents / images (static resources) will still be served up - while this might not be a deal breaker for you - you should be aware of it.
You also shouldn't install Maintenance Mode if you have a complex site where you have replaced the DefaultMVCRenderController - as that is what Maintenance Mode is doing too, and it might get messy - again that is quite an advanced thing to do, so you probibly haven't done that.
More Complex things
If you want something a bit more complex the recommended way to put your whole site in to maintance is to use URL Rewrite rules - there is a good blog post on this (https://leekelleher.com/2009/09/29/putting-your-asp-net-web-application-in-maintenance-mode-using-isapi_rewrite/) if you want to find out more.