differences between 'in there' and 'there'

'There' means 'in that place' but I often hear people say 'in there'. What's the difference? Thanks!

I'm a Brit, but not a teacher.

I would say that:
"there" means, as you say: "In that place/location/area", whereas
"in there" means physically "inside there" - different meaning.

Example: You go to Mary's house.
Someone asks: "Is she there?"
Whether she is inside or outside the house and you can see her, you say: "Yes, she's there".

If they ask: "Is she in there?" and she's outside, you can say: "No, she's outside"

Hope this helps