I agree with Roarmeister almost totally, and when he says:
it provides a convenient holder for the ferrocium rod and that's about it....
That is exactly what I like about the magnesium block too.
I have never actually had to depend on the magnesium shavings to get the fire going and have only bothered doing it to see how well it worked.
Whether that testifies to my superior skills as a woodsman I don't know.
I like bic lighters and candle stubs for fire lighting an awful lot too.