Complex sentence: The "InSight" lander will retransmit its telemetry data soon after landing, so the MarCO test is not critical for the "Insight" mission, but it will demonstrate the new relay system and technology for future use in missions to outer Solar System bodies.
Split into simpler sentences: The "InSight" lander will retransmit its telemetry data soon after landing. Therefore, the MarCO test is not critical for the "Insight" mission, but it will demonstrate the new relay system and technology for future use in missions to outer Solar System bodies.