I looked at one or two of those sites too, and my concern was that jobs tend to get steered towards the lowest bids.
That's not good for anybody. Experienced professionals who will do a good job are never going to bid less than people whose skills aren't up to scratch and who can't get work any other way. That pushes down the perceived value of expert engineering time, which is bad for our profession generally, and at the same time means that customer jobs get done by the people who bid low and therefore are unlikely to represent us in the best light. It's a lose/lose situation.
Maybe you'd get lucky and happen to get a job for a company which ends up offering you repeat business. They're certainly the best customers to have; you get to know the people, the products, their way of working, and what they do and don't expect you to do for them. Certainly if you do a good job the first time, you have a very good chance of being their first port of call when something new comes up.
Most of my customers are people who have seen my work before in some context, ie. former colleagues, manufacturers, subcontractors and so on. I have no web site and I don't advertise, other than mentioning to people in person what I do and handing out business cards.