Creating an account on nostr is costless.. As is sending messages. Imagine a future where spammers are using thousands of bots so send millions of messages. Pow forces your phone to do a bit of work. For you it's a few seconds to post, no problem. For a spammers this can become hours or days of CPU time, not worth their time. Currently we don't have a (major) spam issue on nostr so don't filter on pow. However if everybody gets used to posting pow now, in the future it would be simple to filter VIEWING by pow. We can filter in client. We can filter in relay to save bandwidth, we can have pow only relays that save on bandwidth and storage. Personally I use lowest pow at the moment so posting is instant. If spammers start using a low pow value then I will keep increasing mine to stay above them. It will always be cheaper for me to have higher pow than a Spammer.