Over use is just a factor that most GMRS messages over the airwaves are very short. Usually there aren't long conversations. It's "Honey I'm coming home, over" or "Where are you now and how long will this take, over".
And then the call sign. It's rare when you have 2 people jabber jawing for more than 15 minutes at which time you should announce ID.
Another reason is to get the other parties attention. It's not a telephone that alerts you a call is coming in. It just comes in. And if you aren't paying attention you might miss it. So announcing your call sign, then saying their name and maybe doing it twice because of no response from the 1st try you get multiple instances of IDing. And if the person fails to connect they'll usually sign off with their ID also. And that might just be the last ditch effort to get whoever you want to contact on the other ends attention.