Corporate identity is how a business/ organisation is recognised by, not only, the customers but also the employees and other businesses. For example, their company logo or the colours that they primarily use. This is what people associate the company with. This means that when companies send out emails, letters, posters etc, they become noticeable by the logos etc that are being put on them. By this happening means that the business becomes well branded.
This will then hopefully result in the company becoming widely recognised and growth of the business. Corporate identity is important on documents in order for this to happen. If business documents didn’t have things such as their logo or their business colours on then people wouldn’t make any sort of connections as there’s nothing associating the letter with the business.