That's a great question. When it comes to mapping of the cosmos, we draw many conclusions based on it.

For example, you can read about (as the article says) "THE most precise map so far of dark matter in our universe" here, where conclusions are being drawn based on the mapping.

Imagine for a second that light originating from each nearby galaxy, which goes out in every direction, is bent by the gravity of an object. Some of those trajectories, then arrive at Earth at essentially any angle and any distance away. Essentially everything we see would be optical illusions and "repeats" of the same galaxies or galaxy clusters. We then map this as if they are real, but we are literally mapping optical illusions.

The further away they are, the more likely to be optical illusions. They all exist somewhere, but none where they appear to be. Rather, the light has traveled in bending paths that arrives at Earth after millions or billions of years.

Then, we move forward and draw conclusions about the overall structure of the universe based on these optical illusions. All well known theories out there assume each galaxy we see to be a unique and distinct galaxy, when in reality there is no accounting for gravitational lensing to determine the reality of the situation.

With light traveling repetitively in a very large-scale Figure-8 pattern, it can basically arrive at Earth from any angle and at any distance even if the original source is the Milky Way galaxy itself. Without accounting for gravitational lensing, we would never be the wiser.

Other reasons for it mattering are really about actual understanding. The better we understand how the universe functions in all facets, the more precise we can be in determining anything and everything as well as preventing the distribution of misinformation. Our actions stem from our understandings or misunderstandings. The more in line what is and what we think become, the more we can unlock the doors of actual, tangible, real, fundamental understanding. What lies beyond is, like any step forward in understanding, progress. As Buzz Lightyear says, "to Infinity and beyond!" :)

This post is just one small element of misunderstandings in physics I attempt to address in my posts, but they are all connected. Everything comes back to gravity in physics, as it is the one and only fundamental force of nature. The other three are all demonstrably the result of gravity with certain misconceptions such as those I talk about in the post above recognized and included in consideration.