Why is reverting to old world states mentioned?

[siladu]

https://github.com/ethereum/yellowpaper/blob/f41ec9a6785374c1d9b9e9a064e3ceae1413805a/Paper.tex#L246

[the world state trie] allows any previous state (whose root hash is known) to be recalled by simply altering the root hash accordingly. Since we store all such root hashes in the blockchain, we are able to trivially revert to old states.

[siladu]

Answer from @ajsutton:

This is only possible with a full archive node (in besu, a fully synched 'FOREST' node with no pruning).
In this mode, trie nodes are never deleted and all preimages are stored.
Each new block results in a new trie node for each account in the world state. <-- is this true?
Archive nodes are useful for querying old data, but there's not really a use case for actually reverting to this old state.

[ajsutton]

To clarify: each new block results in a new world state trie but since a very large number of the nodes in that trie will be unchanged from the previous block only the changed nodes wind up being stored so you get deduplication across different world states. Though modern storage approaches generally model things more around storing the account states (the leaf nodes) and are much more indirect about how they rebuild the trie from that. The advantage being that accessing an account can be done with a single look up by the account key rather than having to walk down the trie loading multiple branch nodes along the way.