what are the uses of tweaks in block ciphers? Few block cipher modes has additional parameter , tweaks , especially the ones that are format preserving .

Now the comments section of this blog entry says such tweaks can be used for BIN numbers, expiration dates etc of credit card numbers. 

How can tweaks be used to enforce such checks ? 

what other ways tweaks can be used ?
There are a couple of related concepts here: Tweakable blockciphers and format-preserving encryption (FPE). It turns out that tweakable blockciphers provide a very natural way of obtaining FPE, but they have other uses as well.

As the blog discusses, sometimes we want, say, encrypted credit card numbers to themselves look like credit card numbers. That is, we want encryption to be an invertible function that takes a 16-digit number and spits out a 16-digit number, and this function should essentially look random to anyone who doesn't know the secret key. In mathematical terms, we want encryption to be a pseudorandom permutation on the set of 16-digit numbers.

But sometimes this isn't enough. For various reasons, it's often the case that only part of your credit card number gets encrypted, while the rest (e.g., the BIN and the last four digits) gets stored in plaintext. This creates a problem: what if two cards share the same 8 encrypted digits? Then if one card is known, the other can be recovered by an attacer. Or worse, what if an attacker has an opportunity to build a dictionary mapping 8-digit plaintexts to 8-digit ciphertexts?

This is where tweaking comes in. Tweaking effectively gives us access to a lot of completely independent pseudorandom permutations --- one for each possible tweak value. So if we encrypt the 8 secret digits using the BIN as the tweak, then the attacks above are no longer a concern. From an attacker's perspective, the ciphertext Encrypt(BIN-1, 01234567) will appear independent of Encrypt(BIN-2, 01234567). This remains true even if BIN-1 is very similar to BIN-2, and even if the attacker knows both BIN-1 and BIN-2. And we don't need to restrict ourselves to BINs -- any cleartext data that can be encoded into a tweak may be used in this fashion.

As I mentioned earlier, FPE is just one use of tweakable blockciphers. Because tweakable blockciphers are so much more powerful than (regular) blockciphers, they are a very nice building block to use when designing other cryptographic algorithms (such as OCB).