Its about having a choice. Open floors don't leave any choice for people who like to work quietly away from all noise. Not to mention some of us are little or a lot socially anxious which affects performance when there is lot of foot traffic.

In a place where primary place to work is office, still provides common areas where you can talk to your co-workers and have serendipitous conversations. People who seek social interaction find it anywhere, meetings, lunch etc.

Its about choice.