Containers are a convenient way to remove clutter from the timeline by grouping or nesting elements together. The elements, which can come from different tracks or versions, are treated as one unit yet remain individually editable. Containers behave like any other element but they appear in a separate timeline tab when you enter their editor. You can also add containers within a container.
Containers can also be used to take a clip and matte that are separate and add them to the timeline as a single element.