This prevents buffer overflow errors, but not crashes.
It prevents the OS killing a misbehaving application (which is what is meant by "crash" in that context). This allows, for example, a multithreaded server application to continue even if one worker task indexes an array incorrectly.
It also prevents an equally important problem: silent passing of errors. It's common for out of bounds accesses or stores in C or C++ to just pass silently, potentially corrupting data.