I’m going to add usually. With Mab’s help Alfred was able to leave the island and still had enough power to wig harry out as a Let’s Play Rugby Skeleton shirt. Later in the series, maybe Cold Days?, harry said he could shut down Sharkface and his Outsider band surrounding the island single-handedly now that he was familiar with the islands defenses. So maybe Alfred can’t act outside the island by himself, but with someone else’s assistance it’s power can definitely be used off the island.
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From the start of the books we learn that magic works through your belief in it working. Hell, in Summer Knight Elaine even teases him with Let’s Play Rugby Skeleton shirt: he uses phallic symbols when others do just as well and comments on his phallic thinking. It might be mentioned off-had to present Harry as a chauvinistic manly-man, but it also teaches us, the reader, about how magic works in conjunction with tools.