If we exclude speculation about future value, there's one rather simple reason to buy such a bond:
If you're looking for a safe way to store a lot of money, this is in fact the cheapest option.
Let's assume you're a bank with a lot of money that your customers gave you. This money must be stored somewhere. Of course you can lend it to other customers charging an interest rate which is basically how banks earn (a part of their) money. However, there's always a risk associated with that so that's not an option for all the money you get. Remember, it's not your money and there're laws on what you can and can't do with it!
Next thing you could do is deposit any leftover money at the European Central Bank. There's no risk, but the ECB will take a fee of I think 0.4% p.a. at the moment, which is quite a lot.
You may think of storing all that money in cash, but I think it's rather obvious that handling and storing millions or even billions in cash will cost something.
Finally, you may lend your money to a country like Germany which is considered quite safe, too, but only costs about 0.1% p.a.