In a lot of cases the attacker can only write to a limited range of memory addresses. If that string happens to fall in that range, they can use it to write to

In a lot of cases the attacker can only write to a limited range of memory addresses. If that string happens to fall in that range, they can use it to write to other addresses and/or find out where in memory certain things are stored. So their ability to write to a limited range of addresses can be extended to a larger range.