Multiselect widget improves usability of ordinary multi-select input controls replacing them with a nice and clean widget that allow multiple options to be selected with just the mouse and without taking up more room that you need.
In ordinary multi-select form controls, if users need to select multiple items, then they have to click holding down the CTRL key (for Windows, the command button for 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: