A king's job is to maintain their land and cause their people to flourish. Part of that is ensuring the higher factions, who effectively run the country under the king, remain capable and competent. Matching two such people together makes perfect sense in that regard. Alongside that marriage is an easy way to bestow power, which is his goal here imo. Giving her a position in which she can grow as a magician makes her a powerful asset indebted to him.
The biggest issue I have with this king so far is that he knows his son is an asshat and still hasn't stopped him acting like he has. Beat some sense into him, old man!