The reason they added the extra 10 minutes is to ensure families, infrequent travelers etc. know where the gate is and will have extra time to get onboard if necessary. In reality for most FT members you can show up at the real boarding time or final call 10 minutes before the doors close. If you are in the lounge they will announce the time before boarding starts.

Sometimes with a gate change this is a good time to check the Flight Information Displays and go to the next gate as it gives 10 extra minutes to go to the new gate. I usually allow myself 25-30 minutes before the actual boarding time just in case the gate or concourse changes.