You don’t need to activate it.
If WPBakery Page Builder is included into a theme that you’ve purchased, you can’t update/activate this plugin if you don’t have license for WPBakery. This mean, you need to buy a license for WPBakery plugin.
Of course, WPBakery and other plugins (if there are more of them, like Slider Revolution) will work fine and you don’t need to buy additional license.
But, when there is a new version of WPBakery, you can’t update it until the theme author does not update the theme and version of WPBakery which is included in theme.
On other side, you can buy license for WPBakery (to activate it) and to have the latest version (easy update - no wait for theme author update) but that also can be a problem - what if the latest version of WPBakery is not compatible with your theme? In that case, you still need to wait until the author update the theme.