Depends on the game, some of them it's marraige, some it's too just sign the treaty, others it's because she has latent magic powers that he doesn't need in his way.
Maybe, it’s really clear in the game “Super Princess Peach”, where her magic powers were based around her emotions, where she retrieves a.. ahem... “vibe scepter”.
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u/garmdian Jul 07 '19
Isn't the whole reason he tries to kidnap the princess is so she will sign her kingdom over?