Just think about Air’s anime adaption would have been so much better without those irrelevant routes and characters being adapted into it.
For Air, I think the individual routes don’t help much, since they don’t even appear much, other than in their own routes.
For Kanon, I think the individual routes actually helped the story. Each route made him remember his time in the town 7 years ago, unlocking his memories, which ultimately lead and build up towards the true end.
For CLANNAD, each route was very Family-Themed, okay, at least over 98% of them, plus, you needed the Light orbs in each route from the happy ending to unlock the true ending. The character routes itself was not as relevant to After Story, but shared the message, also helping to actually unlock the true end.
For Little Busters, I think the character routes were beneficial to the main story, but didn’t exactly carry the theme of “friendship” that was supposed to be shown in this VN. The routes weren’t beneficial to the true route/true ending though. It’s like saying Refrain doesn’t involve those routes being beneficial to the story, but Refrain can’t be unlocked without completing the heroine’s routes and fulfilling the regrets that they have, since the main purpose of the artificial world (for the individual heroines), was to fulfill the regrets they had. I guess you can say that Refrain caused the character routes to happen, instead of the character routes helping Refrain. There are messages in each route that help the true end, but you can say the same thing about Rewrite.
For Angel Beats!, it was honestly the only Key-creation where the routes are actually beneficial to the story. The routes actually help with the things and events that happen in Angel Beats. The events that happen in Angel Beats causes the routes to be beneficial and having a bearing on the story. I guess that going into 18-21 back stories is really lame though, when most of us don’t even care for all the characters. With Angel Beats, the individual stories and backgrounds help the main story, and helps the ending to a certain extent. I guess you can say that the ending could still happen even if the individual routes weren’t there, but you could argue that they needed to be there if the characters were there, not to mention Yuzuru’s idea of what he needs to do. Which was to Obliterate everyone, so they wouldn’t be turned into NPC’s, and also so they could finally accept their brutal lives that they lived when they were alive
For Rewrite, I guess you can say that those certain routes helped the true route. I won’t say anything much about Rewrite, since my opinion on it is really biased, since I do love Rewrite.
For CLANNAD and LB, I guess you can say LB and CLANNAD is quite different. After Story and Refrain is basically the main purpose to the routes before those events. But the only difference is that in After Story You cannot unlock it without having the light orbs from each routes happy ending. The light orbs makes the route relevant. but with Refrain, there’s really nothing but foreshadowing, so the individual routes don’t exactly have the same importance as CLANNAD. In individual routes in LB build up for Refrain, yet can be taken out, and the story wouldn’t be much different. Riki would probably be the only different thing/person that changes… The individual routes in CLANNAD don’t build up to After Story, but can’t be taken out if you want to unlock the true ending.