Presumably they could store less of the gold overseas to achieve that particular function but have done some maths that indicate half is an optimal amount.
Can you link to the article? The excerpt you quote suggests that currently, more than half of the gold is overseas and that Germany is ramping up domestic storage (otherwise, it ought to have opened with "by 2020, the Bundesbank intends to store half of its gold reserves in partner banks in New York and London." If they're ramping up domestic storage, it's the sort of news goldbugs will seize upon as it apparently represents an endorsement of gold by the Bundesbank; if they're ramping up overseas storage for the purposes of making that gold easily exchangeable, the opposite seems true.
Edit: I looked up the news and it confirmed what I said above (I wasn't sure because you didn't link to any articles). So the only real answer here is that they've done maths that indicate that having half of that gold available for quick exchange is an optimal strategy for them. What else would the answer be, PIP? Do you feel there is some deep mystery here that needs explaining?
Nothing more, I was just curious as to why a central bank would store half of its gold outside country.
I assumed it was security related as BB said. I know countries around the world use the New York Federal Reserve vault, and Fort Knox to store gold, documents and important artifacts.