What is nodetool cleanup used for?

nodetool cleanup removes data that a node is no longer responsible for, reclaiming disk space after the cluster's token ownership has changed.

• When a new node joins (or an existing node's token ranges otherwise shift), other nodes may keep serving reads/writes correctly for a while, but they still physically hold on-disk data for ranges they've handed off, until cleanup removes it.
• Cleanup rewrites SSTables, discarding rows whose partition key hashes to a token range the node no longer owns.
• It is not run automatically — operators must trigger it manually on the affected nodes after a topology change like adding nodes.
• Cleanup can be I/O and CPU intensive, so it's typically throttled (nodetool setcompactionthroughput-style controls apply similarly) and run during low-traffic windows, one node at a time.

nodetool cleanup my_keyspace

Skipping cleanup after scaling a cluster doesn't cause correctness problems by itself, but it does waste disk space indefinitely, since the orphaned data just sits there unused until cleanup (or a future compaction that happens to touch those SSTables) removes it.