Though Kingdom Hearts is now its own sprawling franchise, the original 2002 release of the first game in the series held strong connections to the Final Fantasy series.
Final Fantasy X had released one year earlier, and was a turn-based RPG, like the nine games that preceded it. Kingdom Hearts was an action RPG that combined characters from Final Fantasy with Disney characters, an odd notion at the time of its release, but one that proved highly successful.
The Final Fantasy games have become more action-oriented since, largely abandoning their earlier turn-based format. The success of Kingdom Hearts as an action RPG could have influenced the direction of the Final Fantasy series.
Source: N4G PC How Kingdom Hearts Has Changed Final Fantasy