The Container widget allows you to create additional rows and columns where you might not otherwise be able to do so on a page. Containers hold other objects or content such as text, media, or even other widgets. Perhaps an easier way to visualize a container is to think of the basic page grid layout, when placing a container within a cell or row/column you are adding another grid layout within that cell or row/column. This will give you finer control over content placement on a page.
For example, if you have a Basic Page and on that page you have two columns with the left column being larger and you would like the right column to act as a sidebar where you may insert additional widgets or formatted content. However, if you had a block of text spanning multiple paragraphs in a row in the left column, the corresponding row on the right column would be of equal size which could only have one widget within that space.
A solution to that problem would be to use a Container in the right column. With a Container, you would be able to insert multiple rows within the same space as the left column row is occupying. This would allow for the placement of multiple widgets and text to create the illusion of a sidebar on the page.