Checkboxs list widget improves usability of ordinary multiple-select input controls replacing them with a more intuitive to use, scrollable checklist that allow multiple options to be selected with just the mouse and without taking up a lot of screen.
In ordinary multi-select form controls, if users need to select multiple items, then they have to click holding down the CTRL key (on Windows, or the command button on Mac). That can get annoying for users, especially with long list of selectable items. For example, see how annoying is to select 3 or 4 countries from the following list.
Select countries from the list: