If you leave a room then you are no longer inside and you can't know what people inside were talking about while you weren't inside. As it should be.

That makes tons of sense for rooms where you're in one or two at a time and have to be actively paying attention.

When it's intentionally simple to sit in 50 rooms 95% of the time, it's pretty stupid for a disconnection to make you lose out on random chunks, or for you to miss events because you wanted to turn off your computer.

When you intend to be always in a room, and the only roadblock is the inconveniences of modern tech, the logical state of "inside a room" should be decoupled from "has an active TCP connection". So sure you'd miss what people talk about if you left, but a disruption to your TCP connection would not trigger leaving.

The only reason why you want to get history is FOMO. A staple in modern software engagement.

Or you're trying to have an asynchronous conversation with someone and don't want to switch to email.

Or you're trying to have a normal conversation on a phone connection, or a flaky connection. Or switch to your phone in the middle.

Or you want to link something to a group of friends and you only want to send one message.

There should probably be a distinction between "conference" and "room", where one is logged and one is not, but it can be argued that logging is an expected feature and access is a matter of the room being private or not. IRC doesn't work like this and many users are not exactly happy, which is why so many run their clients just for logging.