They are used in saltwater setups mainly... if you have freshwater want something that will increase your flow, try a powerhead. It puts out a current in the tank that helps keep all the water moving and stirs up junk from your substrate so your filter can handle it.
How big is your tank first of all? If it really big its going to cost a pretty penny to get a wavemaker for it.
It is correct that they are typically used in saltwater. I have looked into them for my salt tank but take up to much room IMO. If you decide to go with one I would look at Tunze brand. I have see these in action and do the job very well.
Running one on a freshwater tank might not be a bad idea if it helps keeps the wastes suspended for the filtration to pick it up.
I use a vortech mp40w wavemaker on my tanganyika tank. The fish love it. The grazing, schooling and spawning have all increased since I started using it. It wasn't cheap, but it's an awesome piece of gear
Used a sunsun wavemaker in my freshwater tank and my fishes love them, gets them to exercise and keeps them in good shape. Else all they ever do is eat and lie on the gravel bed, making them fat!
Usually people place them at the back, pointing towards the front of the aquarium. So that any gunk from the fishes gets blown to the front and settle in the gravel for easier vacuuming.