It ensure that there is no pending replication of the ADAM database between the connection servers. I took a snapshot with the connection servers powered on once and has to rollback. It was a mess with replication issues and we spent hours on the phone with VMware uninstalling all of the connection servers and adding one back at a time (We had to do this multiple times).
You should do the same thing when upgrading PSC/VCSA if you have more than one.