What are double-ended knitting needles or double-ended crochet hooks used for?

Dear users, dear authors,
what are double-ended knitting needles, which you can simply turn around, used for? Are there also crochet hooks with a hook at both ends? But there are crochet hooks with a double hook at one end, right?

Double-ended crochet hooks are well-suited for Tunisian crochet in the round. They come with the same hook size on both sides or with different sizes (e.g., 4 and 4.5).
You can also use them for regular crochet, though some people find the hook at the end to be a bit of a nuisance. 

Double-ended knitting needles are part of what’s known as a “set of double-pointed needles.”
They’re used to make items worked in rounds: socks, doilies, etc.
Since these needles are short, they’re better suited for projects with few stitches.