The short story is that in this case, “mi casa” means just plain “my house”, whereas “casa mía” means “my house”. Hear the difference? In the second one, you say the word my with more emphasis, as though it were set in italic.
It’s super-duper more common to say “Ésta es mi casa”, but there is room for saying “Ésta es casa mía” (without the la). You might conceivably do that if somebody just said to you that that one was their house.
The stressed construction with mía at the end is the equivalent of leaning on the word in English, like saying “Yeah, well, this one is my house.”
See? That one would be casa mía instead of just mi casa.
You’ll find that Spanish will often simply invert normal word order for places where English more often uses stressed words alone. Inversion here would stress the my part. Sure, you wouldn’t normally do that, but on rare occasion, you just might.