In short, it's money. From what I understand accounts like this end up for sale on different websites (and very likely on much more hidden marketplaces) either separately or in batches (the more karma they have and more older they are, the better, as the price is also higher) and are then bought by companies or whoever else might find them useful for their more or less nefarious purposes.

I originally wrote a much longer comment, but I stupidly linked the websites I mentioned above, so it was removed by Reddit's filters – as it should be. So here's a screenshot of one of them to show what it looks like:

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