The simple answer is that at the time I created the site, I needed to pick up another part-time client, so that I could continue studying and experimenting with my computer vision during the other part of the week.
And by far the easiest way for me to get a contract over the years has been to find a specification and create a demo. But the freelancing sites that have specifications can be quite horrible.
The biggest issue is that they lock you into a very hefty ongoing fee for the privilege of using their site. This easily adds up to thousands of dollars.
Another problem is that specifications and bids do not have very many constraints, and tend to be fairly low quality. There is not a serious effort to ensure that the project postings make sense, or that the programmers have even necessarily read the project description. This means that there is quite a bit of noise that must be filtered through and a fair amount of risk for both parties.
Then there is the issue that this third party controls all access to your money.
This site is run by volunteer moderators and does not charge a fee. By requiring a useful specification and demo, risk is significantly reduced. We also require full names, email addresses, face photos, and have a section for honest testimonials. The testimonials section does not break down to a simple star rating, but rather encourages people to provide details about the interactions.