Why is it so important to understand the evolution of the cosmos?

One could ask the same question about all aspects of science, with the possible exception of those directly related to our survival - notably medicine and high-yield agriculture. With the exception of high tech inventions (many of which are not critical), some of us call the rest of it "recreational science", and there is none more notorious for this designation than cosmology. So your question is far from meritless.

But after a time, it could very well turn out to be the most important thing of all to investigate. Who could know at this point?

As I am fond of noting in such cases:

We can never tell what accumulating knowledge will bring us, but we can be certain what we will get from the lack of it.