I don't understand why they'd do that, I think they would earn more money in a local supermarket or McDonald?
Let's look at salary negotiations when interviewing for a job at McDonalds. The manager of McDolands needs to hire a new employee. He first needs to decide what responsibilities he needs filled and the salary range. We'll assume here that he just wants to hire anyone for any position. To do that he must decide what the lowest level is he's interesting in hiring (let's say a student with no experience), and he must decide the highest level he can afford (let's say a previous McDonalds manager).
The manager now has a salary range (hypothetically) of $7.25/hr to $12.00/hr (range found via Google). He won't hire someone for less than that and he won't hire someone for more. He is going to keep this a secret during the hiring process.
When he interviews people he will ask them what hourly wage they want for the responsibilities they're interested in. He will then weigh the value between what they have to offer with what they are asking for. Any candidates who ask for more than $12.00/hr are not hired, and since $7.25/hr is the minimum wage allowed by law. He will reject any candidates he feels are not worth that amount. He will pick the candidate that offers the best return for his wage. Understand, he won't pick the cheapest unless no one else who asked for more was better than the cheapest.
It's that simple, and it also applies to your situation.
I personally charge much more than them, thus I'm able to earn a significant margin (get a job and hire someone online to help me).
You can't hire someone for more than you make and you want to keep some for yourself. This defines your range. Once you know your range you're in a position to look for help, but the range doesn't change the issue of supply and demand.
Ok. The quality wasn't the top, but I was able to fill the gaps myself. It worked well for me.
You managed to yield a return on your investment in hiring that person. That's all it means. You found a way to locate a supply of workers that fit inside your range. If it didn't work out and it cost you money, then you wouldn't be here asking this question.
What's the point of waiting for a freelancing job online, and only charge it like $10 an hour?
You said "online" and that is the fundamental issue here. It also sounds like they announced their hourly rate publicly. Which means, that you ignored the others who were outside your range. There is no negotiations since you know your range and they've already disclosed their rate.
In the game of salary negotiations the first one to show their cards loses. Always!
You might have had enough range to afford $20/hr but instead agreed to $10/hr. That is not your fault. It's the fault of the candidate who didn't understand the game of negotiating. They should have performed some research to see what the general salary range was for the work they were doing.
The other issue here is the use of "hire me" websites where everyone shares their hourly rate. These websites create a stock market economy where "trends" drive up or down the rates. People are charging based upon their ability to quickly find work using the website, and their rates are tightly coupled to the use of that website.
If they walked down the street to their local customer they would negotiate a completely different rate.
These kinds of "hire me" websites become saturated quickly with freelancers who drive down the economy value by overwhelming the population with a lower standard of living. So in layman terms, you get a lot of people from third world countries. While you might not hire people from these regions you are hiring in their online economy. Which means, that local talent are subject to lower rate competition.