This is a fairly common question that gets asked on the forums and elsewhere and it was asked again today in the comments of one of my videos on the Sniffle Valve YouTube channel.
Let’s expand upon that a bit.
DWB will let you control items that relate to the theme, which forms the structure of your website. These items include the header, menus, footer, widget areas and the like. With it, for example, the color of the main horizontal navigation menu can be easily changed. In contrast, BB is not able to affect this element whatsoever.
On the other hand, with Beaver Builder you can easily and quickly format the layout of the content areas of your site, those being your pages and posts, in addition to adding extended functionality to them. While the theme can affect the styling of the contents, it does not affect the actual layout of the content directly.
This is an instance where a picture (or graphic in this case) is worth a thousand words.