That could be a benefit if the client chooses to keep GBs of events in-memory. Blowater actually plans to "never" delete locally.
Many web clients still don't treat on-device storage the main storage and query from relays everytime.
I believe local-first approach is the only way to have decentralization product-wise and to have fast client tech-wise. Therefore, I like nostrdb's motivation a lot.
But what will be nostrdb's query interface? Function calls? Query languages? Can it answer questions such as what's all my notes which have been replied within 2 hours of creation? I would love to see a nostr "database" that provides graph algorithm queries.