The new multi-device tools within the Site Builder allow you to build and preview Sites as they would display on the various Responsive Device types (Mobile, Tablet, and Desktop)
Using the Device Selection within the Header of the Site Builder, you can toggle between Mobile, Tablet, and Desktop views.
While using the Preview Model/Popup within the Site Builder, you have a device selection to switch between the Rendered Site views for those devices
You have the ability to modify the specific margins (top, right, bottom, left) for a specific module. With the Multi-device tools, these margins are now specific to the Device view you are working within. So if you set the margins for the Desktop view, you can set different margins for the Tablet, Mobile views.
Module Visibility Based on Device
With the new Multi-device views, you also now have options to toggle the visiblity of a particular module based on the Device. This allows you to hide certain modules on Mobile vs Desktop and vice-versa.