>>280388766rather than the part about Touma saving the day for me it's more about the relevance of the arc and how it ties in with the main story
sister's arc in railgun built up nicely and complemented the original index arc in a neat way, it allowed to explore a few facets of Touma, Misaka, Accelerator and fresh perspectives on the events in a way that the main series couldn't
for daihaseisai it established Misaki's character back then when if i recall correctly we were right around the time when NT7 formally introduced her to the main story, then of course there's the whole thing with more level 6 lore and touma's dragons, which (correct me if i'm wrong) was the first ever confirmation of their existence since even the main series hadn't revealed them
you see where i'm going? those arcs built upon and complemented the main series, even introduced / confirmed elements that served as foreshadowing for future events in the main index novels, that's what a spinoff is supposed to do
what did we gain from jailbreaker or dream ranker in terms of tying in with the main series, providing exposure of things that the main series did not have the time/chance to address, foreshadowing for current or future arcs or exploring different perspectives of past events?
yeah, you're right, jackshit