It's basic research and has no immediate application for normal life if this is what you are wondering about.

For astronomy, it is giving us a completely new way of observing the universe. Most other ways were based on different ways of observing electromagnetic radiation.

https://en.wikipedia.org/wiki/Basic_research

The justification is knowledge.

Primarily it means there are more potential events to observe, which again leads to better statistics and hopefully more of the interesting events.

Better statistics can help guide us towards a viable theory of quantum gravity, as well as improve our understand of what happens around black holes and neutron stars.

I'm not a scientist but: I believe it provides access to the only real "data" on events that actually occurred very large distances away. This data is valuable in aiding our understanding of the early universe e.g. the big bang theory.

Just creating these machines moves science and engineering forward.