1. Everything that humanity discovers helps humanity, because knowledge is something we value.

2. Things it helps us discover about the universe may lead to new discoveries in physics (consider how astronomical observations helped in discovering and confirming relativity), which ...

2a. ... are in themselves a gain for humanity (see #1), and ...

2b. ... may lead to technological advances (e.g., relativity led to nuclear power and nuclear weapons, which may or may not have been a win for humanity overall but are certainly major advances in what humankind is capable of doing; also, e.g., GPS accurate enough to be useful).

3. Any discovery may end up being valuable in ways no one ever suspected ahead of time.

4. The project will provide employment for many scientists, and thereby contribute to whatever further work they and their institutions do later, which will be valuable in the same sorts of ways as the Einstein Telescope itself.

5. The project will provide a precedent for government spending on science, which there should be more of for the reasons given above.

(Does it help humanity enough that if we had to choose between this and feeding starving people in the world's poorest places we should choose this? Probably not. Does it help humanity enough that if we had to choose between this and buying a few more fighter jets we should choose this? Probably. In practice, neither of those is likely to be a decision anyone's making.)