Its for long urls links that sometimes break the layout of
websites or is just hard to remember or tell others (not on
computer, maybe over the phone etc)
From WIKIPEDIA: Short URL aliases are seen as useful because
they're easier to write down, remember or pass around, are less
error-prone to write, and also fit where space is limited such as
IRC channel topics or email signatures. Also some email clients
impose a maximum length at which they automatically break lines
requiring the user to paste together a long URL rather than just
clicking on it. A short URL alias is much less likely to become
broken.
