LogFAQs > #978805047

Basically at the end of the day with minimal enough spoilers basically all the past games add up to one key event:

Seven Warriors of Lights versus 13 Warriors of Darkness to clash to create the X-Blade, the one weapon that is capable of opening and controlling Kingdom Hearts. Thats really what it boils down to, and yes there are quite a lot of characters that if you only played main number games you might be confused on who some of them are and what their relationships between each other are. Hence why those side games are more a part of a bigger narrative.