A current source regulates the current that flows through itself by adjusting the voltage across itself. Regulating a current may mean creating it if it does not exist (which is why current sources are not meant to be in an open circuit as they will try to force one by increasing their voltage until it is maxed out or it breaks through air). It is normal that the same current goes in and out, since the current goes through it - the source is in series with that branch of the circuit.
Current sources can be used to create a current, or to ensure the current in that branch of the circuit is at a known value. When you think about it, a simple resistor with a voltage source can be a current source - a not particularly efficient or compliant/robust one but it still is. If a current source is used to create a current, this current can be injected in other branches as currents add up at each circuit node.
Voltage is generally used to transport information, but current sources are everywhere. From biasing to driving LEDs.