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A pillar page covers a subject broadly. Cluster pages cover its parts in depth. Every cluster page links to the pillar and the pillar links to each of them.
The linking is not decoration — it is what tells a search engine these pages are related and which one is the entry point.
It demonstrates depth on a subject rather than isolated pages about it, and it gives internal linking a sensible structure instead of ad-hoc cross-references.
It also helps readers, which is the more durable reason. Someone arriving on a detail page can find the overview, and vice versa.
As many as have genuinely distinct questions. Splitting further creates duplicates.
Useful on its own rather than long. A page of links is thin regardless of length.
Yes. One direction only does most of the work of neither.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.