Visibility option of fusion builder allows you to control the visibility of elements in different size devices(mobiles, tablets and desktops). While FBMLC allows you to customize the appearance of fusion builder elements in mobile devices. It will allow you to have multiple columns in a row in mobile devices. For example, with FBMLC you will be able to show 3 buttons in one row in mobile devices. Also with FBMLC you create mobile layout of your page as a separate page.
Let me explain with one more example, suppose you have a page showing 6 content boxes in 2 rows (3 content boxes per row). In FBMLC settings you set the upper limit for the mobile layout to 800px and lower limit for the mobile layout to 360px. Then you create a mobile layout for your page and in that mobile layout you add 6 content boxes in 3 rows (2 content boxes per row).