RadDomainUpDown in Telerik UI for WinForms is a combination of a text-box and a pair of moving up and down buttons to navigate through a limited selection of options. This control may save you some screen space since it occupies the space needed for a standard text-box. However, in addition, it allows the end user to select one of a variety of several items.
A common use-case is to build an input form for filling personal information. One of the required fields is the nationality. RadDomainUpDown is suitable for introducing the countries options if you don't want to allocate a lot of space on the form.