The standard multiple select list can be difficult to use, if your users aren’t very technical. Most of the time they won’t know that you have to hold down command or ctrl if you want to select multiple options. Also, once you have more than 10 options, the select list becomes very difficult to use. One wrong move and you can loose all the selected options.
The Multiselect module defines a widget that allows users to move selected items from one box to another. Visually this helps users to see which items have been selected. The widget can be used on a bunch of field types and it can be implemented as a form element using Form API.
drush dl multiselect