In the sphere of "talks," I know TED puts a big round carpet on stage that you're supposed to stay on while speaking. More generally for "carpets on stage," I've seen some smaller music venues and studios with carpets to make a nicer area for performers and to absorb unwanted sound (in the case of studios)

My best guess is an acoustical reason. The floor might reflect the speaker's voice in ways that muddle it for the audience, so those carpets are placed in front of them to kill that reflection?

Perhaps glare? They are getting hit pretty hard by that overhead, might have washed out the projector if they were left to reflect off the floor.

I'm the organizer of this conference. The stage is very flexible and has many opportunities for trap doors etc. It's honestly just a little creaky.

We've seen other benefits like folks have mentioned. It draws speakers out from behind the podium.

The theater we use is popular and we often didn't have the stage setup until 2AM the night before a 9AM start time. As organizers/hosts we couldn't be up that late.

Sometimes you come in and the stagehands at the venue read the notes about putting down the carpet and they decided to put two carpets instead of one. And the carpets are secured with gaffers tape. And you don't really see an issue with it so you leave it be.

shrug

Stuff happens.

Lis' talk is excellent though.