Compared to the PoW solution, the stamp solution not only solves the spam problem but also generates income for relays. This allows relays to operate sustainably. nostr:note1dlll08xzrygdw9jhw3gql24mgsh79ncq3d45e0fmklcy007257hs0hchux
There's more to that. If spamming gets lucrative spammers would just deploy ASICs which most people don't have. So if the difficulty is low spammers can just spam and if it's high then normal people can't use the network either. Money solves this problem because everyone becomes specialized.