When using shared memory, why should we care about creating a key

key_t ftok(const char *path, int id);

in the following bit of code?

key_t key;
int shmid;

key = ftok("/home/beej/somefile3", 'R');
shmid = shmget(key, 1024, 0644 | IPC_CREAT);

From what I've come to understand, what is needed to access a given shared memory is the shmid, not the key. Or am I wrong? If what we need is the shmid, what is the point in not just creating a random key every time?

Edit

@link text one can read:

What about this key nonsense? How do we create one? Well, since the type key_t is actually just a long, you can use any number you want. But what if you hard-code the number and some other unrelated program hardcodes the same number but wants another queue? The solution is to use the ftok() function which generates a key from two arguments.

Reading this, it gives me the impression that what one needs to attach to a shared-memory block is the key. But this isn't true, is it?

Thanks