I've tried searching for this, as I know it's quite a simple question, but I'm probably not using the correct terms.
I'm relatively new in learning German, and I often see things like the following: "Andreas? Der ist da."
I know that in general, once you have introduced something (e.g., Andreas) you can use its definite article "der" to reference it after that. But is it more correct than using "er" in this case? Coming from English, "Er ist da." seems natural, but I can't understand why you would say "Der ist da."