Prior to this, users had to choose between either voice or video when making a call, and swapping between meant ending the current call, which can be disruptive and inconvenient. This change means that users now have the freedom/flexibility. However it should be noted that at the moment the feature seems to be only available on the Android version of the beta.
This means that iOS users probably won’t get to see the changes until it is live, which at this point in time is unclear when that will be. It works pretty straightforward, and when users want to make the switch they’ll just need to tap the button and a request will be sent to the other user, since obviously no one likes to be surprised by having their cameras suddenly turned on.