For forage fish, underwater habitat offers protection from predators. Habitat gives fish a place to congregate, which appears to reduce stress levels in fish, making them less vulnerable to disease, healthier and more prolific on the spawning bed. For predator fish such as largemouth or trout, habitat is equally beneficial. There are two things nearly every animal needs - food and shelter. They position themselves in areas where food is prevalent and try to ensure that they themselves don't become lunch.
Artificial habitat comes in all shapes and sizes.
Christmas trees, old tires, concrete cinder blocks, just about anything you can imagine is used to provide habitat. There is even a government program that dumps old tanks and airplanes into the Gulf of Mexico for fish habitat. Open any pond supply catalog, and you'll be sure to find a handful of man-made fish attractors. They are easier to install than concrete and other bulky materials, and unlike aquatic plants, do not tend to grow out of control. This makes AquaMats and similar products the next generation in aquatic habitat.