As opposed to most other text input fields you encounter in the wild, credit cards (and phone numbers) have a singular canonical format. As long as you don't limit HOW it's entered (or pasted), it makes a lot of sense to coerce the input into the canonical format.
At least for the US, the issuer of a credit card (and thus the format) can frequently be determined from as little as the first digit. For instance, nearly all card numbers starting with '4' are Visa.
Most fields don't reformat and add characters like spaces as I type. I personally find it a bit jarring. I suppose reformatting it one the box loses focus isn't as bad, but I'd still prefer you just keep what I type.
Try moving to a country that splits phone numbers different that what you are used to. I grew up with 3-3-4 but live in a country that splits 2-4-4. Subtly annoying.
