Good question! Hm. Well, if there was no internet during install, the checkbox effectively does nothing. So it’s often best to make sure you have a network cable or tether connection during Ubuntu install when possible.
But yes, during the install wizard, a user must explicitly check the box during that installation. If it wasn’t checked. Then, with internet access connected, you can go to Settings → Additional Drivers or by installing the required package (in this case bcmwl-kernel-source) once the system has internet access.