Your **super super secret key number** can only ever be "seen" through some "mask"

nsec is one kind of mask, the default for nostr keys.  hex is another.  binary, english, etc, other possibilities.  Humans in our heads default how we mask stuff to base 10, decimal, in our routine thinking & daily affairs, because of fingers I guess 😏.  For nostr, nsec makes more sense for a bunch of reasons

It doesn't matter which mask someone sees the ***I can't stress how secret it is*** number through;  they can always translate it once the secret is seen through any encoded form!  English to binary is how computers work today after all, they can do a lot more "mask switching" than that 😉

Keep the secret, secret!