In a flowing stream, this behavior leads them to hold position in a stream rather than being swept downstream by the current.
In fish that exhibit rheotaxis, it can easily be demonstrated by creating an artificial current within a tank using pumps. No matter which way the pumps force the current to flow, the fish will immediately turn to face the oncoming current.
Some fish will exhibit negative rheotaxis where they will avoid currents. Some zooplankton also exhibit positive or negative rheotaxis. Positive rheotaxis is found in most major aquatic invertebrate groups.