The reasons to check-raise are exactly the same as the reasons to bet, and there are only two:
- For value.
- As a bluff
There is also a continuation bet, or a bet to take down dead money, but that really should be viewed as a combination of both a bet for value and a bet as a bluff.
If you consider a check-raise in this context, a few tactical patterns emerge.
From the perspective of a value bet, a check-raise is designed to get the most money in the middle as fast as possible. This is often done when you are certain that an opponent behind will bet if you check, and that by checking you do not put yourself in a difficult position where you have more opponents than you want. Because of these limitations, check-raising for value is rarely the correct play. Indeed, as I have said elsewhere, the best way to make money with a good hand is to bet, bet, bet.
Check-raising as a bluff is somewhat more common, but still unusual, and very strongly relies on your ability to understand your opponent. In particular, you must have a good understanding of both the relative strength of his hand and how he will react to a check-raise. You also need your check-raise to tell a convincing story. Since the default reason for a check-raise is generally for value, your check-raise must plausibly project great strength. If you have acted weak the entire hand and there is no indication on the board that your situation has changed (ie check-raising a 4H when the flop came KSTS2C), your check-raise will not be believable and often be perceived as a bluff.
As far as the ethics of a check-raise is concerned, there are no qualms: a check-raise is not an unethical play any more than any bluff of value-bet is unethical. Some house games impose a "no check-raise" rule and that's fine. Obey that rule. These games often try to cultivate a friendly atmosphere -- don't be That Guy who ruins a perfectly good time.