its just a neater way of connecting the alternator to the electrical system, many cars are missing this i think because people broke them changing the battery or unscrewing a frozen nut on the stud to pull the alternator. usually people put the wires back together with a bolt and nut then electrical tape the entire thing into a huge mass of insulation.
the wires are color coded to match so. basically the junction just isolates the wires from touching body ground. there is actually 2 ways the harness could plug together. i remember when i got my car some low IQ mechanic had taped one end of the harness that was suppose to go on the junction, and shoved it under the battery then connected the alternator harness to the solenoid and had a spliced wire running down the rest of the electrical harness.
when i redid my engine bay harness i fixed everything and got a junction from a junk car and put it back like factory.