I think comets contain molecules that can lead to the creation of life, if they hit a planet in the 'goldilocks zone' where water can exist and accumulate in the liquid state, provided that the planet was not using a sponge or other prophylactic measure at the time of collision.
We did not plant any evidence of life on earth on comet Halley
when it passed by. Extraterrestrial civilization might not care to
do that just like us.