Skype for meeting broadcast. Office 365. Blocking all domains?

Hey Guys, 

Going through some documentation here: https://docs.microsoft.com/en-us/skypeforbusiness/set-up-your-network-for-skype-meeting-broadcast/set-up-your-network-for-skype-meeting-broadcast

In this documentation its saying to block all domains except for the these 4: 

• noammeetings.lync.com

• emeameetings.lync.com

• apacmeetings.lync.com

• resources.lync.com

My quesiton is if you block all domains except those for Skype for meeting broadcast, wouldnt that break all other external communications? And why do you have to block any domains, or add the above domains? Couldnt you just allow the broadcast meetings to run with the default domains in place (assuming all domains are allowed). ? 

Thanks, 
Robert