In theory it's good for spam prevention. Publishing a normal amount of notes is not computationally expensive but spamming is. However, I don't see the point if you can set it yourself?

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You can read more here:

https://nostr-nips.com/nip-13

I guess some relays or clients could enforce a strict PoW requirement for notes. To get your notes accepted you would set a certain difficulty.

Thanks. Yes, you can set the amount in noStrudel and Iris, the ones I prefer at the moment.