SemiSerious, you can use straight fiat on the Populous platform, but using PPT Token gives you advantages.
The idea behind using blockchain technology is to keep everything cheap(no banksters), open, truthful, honest and verifiable for all parties.
edit:
If you know absolutely nothing about cryptos,
Step 1) sign up at kraken.com and/or coinbase.com
Step 2) transfer fiat to kraken / coinbase to buy Ethereum and/or Bitcoin
Step 3) come back here when step 1 and 2 is done for further instructions

You can trade Ethereum for PPT Tokens at etherdelta.com
https://etherdelta.com/#PPT-ETHvideo below explains how to use etherdelta, it's really easy.
https://www.youtube.com/watch?v=PzpeEuPMsL8I'm quite familiar with Etherdelta, but my point is say a guy with 10 million dollars is seeking yield and bank interest is close to nothing. He is interested in trading invoices on Populous platform to earn interest per year. What is stopping him from bypassing the PPT token alltogether? For instance, the Ethereum platform would not work without paying Ether. Is the PPT token crucial to the Populous platform or is it an afterthought?