The reality is that there are a number of good reasons to be doing this. There are an enormous number of tech spinoffs that result (you're using one of them). Medical, industrial, informatics, etc - we're solving problems (out of necessity) that the rest of the world hasn't even run into yet. The data rate from one detector is greater than every human being on Earth having 20 phone conversations at once.
We're one of the reasons that the internet was developed to its present form.
But mostly that's good for telling politicians why to fund us, so they can do cost-benefit analyses with Beltway bandits and justify the expenditure to the OMB without being scalped. The real reason for all this is
We Are Not Human Beings If We Don't Explore.
We become sheep. We surf the web and watch network TV and do stuff that is fun but stagnant. Or stuff that is not fun and even more stagnant.
Poking at the fundamental levels of our knowledge is quite different from Googling the result - and takes time, money and expertise. These questions we're asking right now - we're asking them because we hunger for the real story. Fortunately, it's relatively cheap to do so. 5 billion in national terms is the price of a nice dinner in personal terms. In international terms, it's chump change. We'd do it cheaper if we could - but it's hard to examine things a octillion time smaller than you.
We pay it - though there are worthy causes that could benefit from that cash - because succumbing to stagnation is to deny who we are, to turn our backs on the contributions of the giants on whose shoulders we stand, and to declare as a civilization that we're done looking forward - we're happy with what we are now. We roll over and go to sleep.
I stand for something better.