Why? And how do you know when boarding begins?

The "boarding at" time is an indicator when approximately staff members will show up who then eventually will trigger the boarding process.

I have to admit though that recently, I was on an LX longhaul which indeed has left the gate on time and as a consequence, I was one of the last to climb aboard. That didn't happen to me in ages ... showing up 5 to 10 minutes after the official boarding time always was a safe bet in my book ...