User blog comment:B214/Question and Answers 6/@comment-26293136-20161211205426/@comment-10330822-20161213103124

Humans can learn magic of other races. In fact, that is the whole attraction of human magicians contracting themselves out to devils. They get protection as well as access to the devil's spells to add to their knowledge of spells. Likewise, the devil can use the magician to learn non-demonic related magic.

For example, Issei could teach Le Fey his Dragon Shot, Dress Break, and Bilingual abilities, and she could teach him stuff like Fairy Magic, Black Magic, White Magic, and Norse Magic. That is the big advantage of their contract. Whether Issei has the talent and intelligence necessary to learn these spells, is another story. Le Fey may get more out of the contract than Issei, since Issei probably can also get her access to the Gremory's spells, who are a magic-focused clan, especially if he marries Rias. Having a contract with a future lord of Gremory, that's like hitting a gold mine.