Junctions allow player to send items across other conveyors or conduits without mixing. This is useful in situations with two different conveyors or conduits carrying different materials to different locations. There are two types of junctions, one for intersecting conveyors, and another one for intersecting liquids.
Junction blocks can be placed at the intersection of conveyor belts or conduits. However, liquid junctions are unable to transfer items, and regular junctions can't transfer liquids. This makes it impossible to cross a conduit and a conveyor using junctions. Junctions also can't be rotated.
When space is limited due to a nearby conveyor and you want to drill ores from depots that are adjacent to your conveyor, and you wish to avoid contaminating the conveyor, the junction can prove to be useful. Simply put the junction at the point of the conveyor where the drill is outputting, and the items from the drill will try to pass through the junction instead of the conveyor, and the conveyor will not be affected. The same effect can be achieved with Tunnels, but junctions are easier to use for this purpose.
As you grow more advanced with your use of junctions, you will be able to create larger and more complex setups. Below is an example of this.
As the animation shows below, item transfer by junctions is not instantaneous. The 10 junctions transferred the iron at a slower speed than a conveyor. As shown, this also can be applied to Tunnels.