Why Do People Use These Cheques?
Sometimes people need to pay things with cheques, but they don't always have the money to pay for it. Since that is essentially a credit card's purpose (to pay for things when you do not have the money for it), these cheques are meant to be a way to charge your credit card for things that require a cheque.
Anything can be paid with these cheques, including rent, bills, groceries, what have you. There are several places that do not like taking credit cards, and these cheques are your way around that.

Should You Use Them?
However, just because these cheques are available does not mean you should necessarily use them. Credit card cheque books are one of the primary ways that people find themselves in more debt than they can handle.

• Having these cheques makes it too tempting to use it to pay for some large expense, like a computer or rent. People have it in the back of their minds that whenever they use their credit cards they are going to experience some debt. But cheques do not have that association, so many people make the mistake of using them in situations that do not warrant it.
• Some of these cheques have unusual rules or interest rates that make them a worse deal than simply using the credit cards instead.

Also, there is something inherently unsafe about these cheques. They are always approved, so if you accidentally keep these cheques out in the open, someone can take them and spend then at their leisure.