Why do you need to cool the filling of a meat pie before adding to the pastry?
You may already know that pastry making requires everything to be cold - the fat, the water, your hands, the board, everything. This is to prevent the fat from melting into the flour prematurely, which results in cardboard-like pastry. When the fat remains cool until cooking, it melts into layers in the pastry as it cooks, making it nice and flaky.

If you put hot filling into your pastry case, you will melt some of the fat prematurely and so the pastry won't be as good as would be if you let it cool.