The mobile games can go screw themselves, but the other handheld titles are actually decent. They filled the gaps between the main titles. They weren't spin offs, just extensions to the story that helped set up the next entry.
Birth By Sleep takes place 10 years before KH1 same as Crisis Core takes place before FFVII. It's also a dark story that sets the stage not only for KH1, but also KH2 and 3 with the main villain and how the organization got started. Chain of Memories sets up why Sora is sleeping and why Roxas took his place, then 358/2 Days tells the final year of Roxas existing up until the final week of his life at the start of KH2 right when Sora wakes up. RE:Coded was just a remake of a Japanese cell phone game that more or less recounts events of KH1 but it doesn't have anything substantial to say other than Sora is the chosen one. No really. RE: Coded is a complete waste of time unless you want to play a bunch of different mini games in short bursts. 358 Days is AWESOME though! You get to construct your own Keyblade with stuff you equip to it that not only changes it's shape but also it's design and elemental magic ect. You can boost your HP, your speed, attack power, defense, ect. It's sort of like how you rearrange your equipment in a western RPG. Storywise, aside from pulling Xion out of the blue as the sudden 14th member of the organization which has been 13 members up to that point, it pulls a Bleach Memories of Nobody with a girl who isn't meant to exist who becomes the main character's gf and eventually disappears forever with no memory left of who she was. Nomura had to have been a HUGE Bleach fan. Dream Drop Distance is the lead up into KH3. There's a weird mechanic where you swap between Sora and Riku because they exist in parallel worlds to each other and it's timed when they fall asleep and wake up. It has a lot of big reveals like how the main bad guy has been planning this from the beginning and the REAL Organization is being formed with Sora more or less a target for joining them. The reason everyone has a giant X on their outfit (other than Nomura being Nomura with weird fashion choices) is that the X allows the bad guy to track Sora everywhere he goes. That's why he ditches the X's in KH3. Also there's time travel and the bad guy AKA Xehanort, the old man from Birth By Sleep has been planning this from the start and used time travel to make an endless loop like Garland in FF1 where when he's near death he jumps back in time to meet himself as a teenager and tell him everything. Also the new organization is made up of Xehanort through time and other Organization members who gave themselves up to experiments to become clones of Xehanort. Xigbar is one of them. It's a lot like Hojo with Sephiroth clones. Vexen of the organization is exactly like Hojo and even shares the same voice actor. That had to be intentional. Anyway it all wraps up in KH3 where Sora has to stop Xehanort in the same place where Birth By Sleep ended, the place where the Keyblade War happened in the time of the lost masters. Think of Keyblade weilders as Jedi from Star Wars and yes there is even a dark side. Xehanort fills the role of Palpatine who more or less turns Terra (Not FF6 Terra) into his Anakin Skywalker turned Darth Vader. Also they use red lightsabers. Yes, this is real. This is what happens to Terra's body after Xehanort fuses with him.
View attachment 100825