A topic cluster is a deliberate content architecture, not a tag. The pillar covers the topic broadly; each cluster page covers one sub-topic in depth; every cluster page links to the pillar and to a few sibling cluster pages.
Search engines (post-2018) reward topical depth more than they reward individual keyword optimisation. A site with 30 deep, well-linked posts about email marketing will usually outrank a site with 100 shallow posts spread across 20 unrelated topics.
The discipline part is editorial planning. Before writing the cluster pages, map every sub-topic the pillar should cover, then write to that plan. Without a plan, you end up with 12 cluster pages that all overlap on the same 3 sub-topics and miss the other 9.