Without knowing the actual implementation:
I’d wager comments are preserved and don’t get cleaned up over time. Because if content gets deleted the instance has to federate the deletion to other instances to clean everything up (like an event system). If the instance just vanishes there would be no deletion request happening.
If deletion was automatic when an instance goes down we’d have already lost thousands of comments due to the outages lately :)
I personally like to keep it on. Most of my messaging is with family and friends and it’s good to know if someone read or hasn’t read my message.
Especially if things are time critical. Picking someone up? Asking if they need anything from the supermarket? If I see that they read the message I know that they are going to reply in a moment. If they didn’t even read the message I won’t have to wait around / can guess that they are currently in the car or wherever.
Sometimes you also have a spotty connection, so the received + read receipt can tell you if they actually got your message.
In general if someone sends me a message and I read it… I’m going to fucking reply to it (if I’m not super busy, and even then I might send a quick message back). I seriously don’t get people who just leave things on read and then forget about it.