Except it's not, if you look at the broader Ethereum & smart contract ecosystem. It's also for decentralized organizations (Aragon), incentivizing distributed storage (Filecoin/IPFS), pricing ad markets (Brave/BAT), supply chain management (VeChain), managing ownership (NFTs), and project governance and voting (many tokens).
This is like saying "The Internet is for porn" because that's your only exposure to it. Yes, it's the most common early use - but it's very far from the only use.
There has to be a coin attached to it because that is how you incentivize actors within the ecosystem to take certain actions (eg. "share their hard disk space" for FileCoin or "view ads" for BAT).
The central premise of these projects is that humans make terrible decisions, markets make good decisions, so let's replace humans with markets whenever possible. Right now, some exec at Google determines how many ads you see on the Internet. The premise of Brave & BAT is that you decide whether you want to see ads on the Internet, you get compensated for viewing them, and if enough people decide the ads are not worth their time, they'll turn them off and drive the price of BAT up enough that people do want to view them.
Right now, some exec at Amazon decides how much you pay for S3. The premise of FileCoin & IPFS is that lots of ordinary home users have spare hard drive space, and they should be able to be compensated for renting out that space to projects that need lots of distributed storage. The market price of FileCoin is that which equilibrates demand for storage with supply.
The requirement is compensation. You could just pay them through PayPal. There's no need to have them do a stock trade for somebody else in order for you to pay them for S3 space.
This is like saying "The Internet is for porn" because that's your only exposure to it. Yes, it's the most common early use - but it's very far from the only use.