In order to test cables with a power meter and source or with an OTDR, one needs to establish test conditions. The test conditions are similar to how the actual cable plant will be used when communications equipment is connected (see below.) For insertion loss testing, this requires reference launch jumper cables to connect the test source to the fiber in the cable under test and receive cables to connect the fiber optic power meter. For OTDR testing, this requires a reference launch cable to connect the OTDR to the fiber in the cable under test and a receive cable at the far end of the fiber.
This video covers why one needs reference cables, hot they are used for both insertion loss and OTDR testing, how they affect measurements, how to choose them and care for tehm.