Simple; to archive and preserve cassette recordings (like what I've been known to do), as kfbkfb stated. Purchasing secondhand also has the advantage of keeping one more better-quality deck (like my workhorse Technics unit with MR head) out of the hands of hipsters, who will invariably bin it once tapes go out of style amongst their nonconformist cell-phone addicted peers. They can have the Chinesium all-plastic machines designed to break after a month.
I don't understand why the question, nowadays, would even come up.....