Prevent application to make silently install file(s) or making changes
Request install will require the user to accept the install, this permission is probably used for two things on Facebook specifically   


Redirecting users to install Messenger   
Redirecting users to install Updates to Facebook   


I understand your concern with allowing an application to install other applications without notification, but this is not possible, the permission only allows the application to redirect the user for installs, you still need to accept the installation.

You can prevent the application by using a permission manager, some of them recompile the application and completely disable the permissions, other use AppOpps to disable the permission.