Moving Walkways (formerly called Conveyors until Update 0.12.3) make crew move faster in one direction but slower in other directions. Speed is increased by 50% in the primary direction, 0% when walking across it, and decreased by 50% in the opposite way.(verification needed) When congested primary speed is .6x, which is twice as fast as normal congested and faster than walking through components.
When walking onto the conveyor your crew will be slowed down. This can be mitigated by having corridors on exit.
An example usage of conveyor walkways
Conveyors can be very useful when moving crew and supplies from one location to another over a long distance, such as from an area dedicated to crew quarters to the weapons, thrusters, and shields. However, putting crew quarters closer to where crew is needed is generally preferable.
Conveyors can be used in small loops to greatly speed up the supplying of energy and munitions to weapons. This can more than double how quickly they are supplied, as you not only get a speed boost from the conveyors, but you also eliminate a lot of crowding, as crew will tend to follow the loop rather than return along the path they came.
Railguns have a large number of ports for energy that don't actually consume much energy individually, and as such, conveyors running parallel to the railgun means you can supply the necessary energy more efficiently and with fewer reactors.