You're missing the point of what a blockchain is good at vs what it's not. Blockchains are good for assigning ownership. For networks which don't need to track ownership, of course you shouldn't use a blockchain. Bittorrent, Freenet, etc do not track ownership as infinite copies of something is not a problem, but a feature.
But for networks which do, you may want to. For example, only one person may control this name, this domain, this tradeable asset, etc.
Blockchains are an addon to distributed models because they allow a distributed way to prove ownership of something. Say I want to look someone up by name instead of by cryptographic hash, how do I do it? You can't register a name in a Freenet or Bittorrent model.
Beyond ownership, an important class of use cases is promises. Without blockchains, it's hard to determine how well a potential trade partner follows through on their promises without a third party that maintains a reputation history. With blockchains, people can provide cryptographically signed promises that give their trade partner the ability to publish their satisfaction or lack thereof. You can retrieve a complete reputation history without a middleman, which is a brand new thing.
People are using blockchains for too many things, but that's healthy. It's the only way we can find all the things they're useful for.
But for networks which do, you may want to. For example, only one person may control this name, this domain, this tradeable asset, etc.
Blockchains are an addon to distributed models because they allow a distributed way to prove ownership of something. Say I want to look someone up by name instead of by cryptographic hash, how do I do it? You can't register a name in a Freenet or Bittorrent model.