Actually, the system is just basically a marketplace. (The company behind Purse.io makes their money from taking a percentage of each transaction, not from the actual buying and selling.) That is not to say that people don’t make money speculating on Bitcoin (or other virtual currencies like Ethereum), but they mainly do so on Bitcoin trading platforms such as cex.io, Kraken, btc-e.com, GDAX, etc. The overhead is a lot less buying and selling on places like those.
The way I (think) it generally works on Purse.io is that people are paid in Amazon gift cards to do tasks by places like Amazon Mechanical Turk. They would rather have something spendable, so this is how they convert gift cards into Bitcoin. (which is basically as good as cash, and depending on your local currency in your part of the world could be a lot better than cash)
In my experience, Bitcoin’s value is becoming more and more stable over time (less wild swings than the early days) as more and more people adopt it for regular use, but it is definitely more volatile than other currencies, like the US dollar. However, a $30 swing over a few days is not that much. (It comes out to less than 5% of the value of a single Bitcoin) For comparison, the British pound lost more than 10% of its value in less than 24 hours after the Brexit vote and that is a much more established currency. Also, you can buy any portion of a Bitcoin; You don’t have to buy them in whole amounts. I will often buy, say USD$100 worth of Bitcoin, no matter what the price is.
Also, the “value” of Bitcoin as a currency is somewhat more than whatever the current trading amount is in my experience. This is due to its buying power I think. (also the value of increased anonymity vs credit/debit cards) If you were to sell Bitcoin for straight cash somewhere like LocalBitcoins, you would expect around an 8% (or more) markup for the convenience for your trouble; Also, like the discounts show on Purse.io or other sites (I often get 40-45% off for, say Panera or TGI Fridays gift cards if I feel like eating out instead of eating Soylent if I pay in Bitcoin) allow for “cheaper” transactions if you buy things with Bitcoin instead of credit/debit cards. Sure, it is some trouble converting your local currency (USD$, CAD$, £, or € to Ƀ for most on here, likely) because you have to either link a bank account and wait a few days or pay a higher fee for purchasing through a credit card to get it immediately, but I have found it worth it to keep a small amount ($100 or so) in a Bitcoin wallet at all times for purchases. If I have a big purchase coming up, like a laptop or something I can transfer a large amount in anticipation of making that purchase. It is worth the trouble for the amount of savings, in my experience.