The vendor (Redhat / Novell / Canonical / ...) do their own QA, then release. Partners, customers, community all report bugs and these get fixed, so the kernel theoretically gets more and more stable - each customer benefiting from the bugs found by other customers/partners/etc.
The common -stable kernel series is an attempt to provide these benefits across distributions.
There are other (claimed) benefits too such as third-party certification, and incorporation of "valuable" out-of-tree patches.
So I don't think there is "only" one benefit - there are several. I wouldn't claim to suggest what the "main" benefit is though.
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