As the data keeps streaming by, sniffers capture each packet and decode them if needed showing the value of various field in the packet. The content then gets analyzed as per the specifications. The analysis helps in determining if there are unusual packets. This helps in maintaining effective data transmission.
A packet capture can be used in two ways- legitimately and illegitimately. In legitimate use, sniffing identifies the data and transmission errors to ensure an efficient network communication. However, in an illegitimate scenario, a disgruntled employee can use the data to actually ‘capture’ the company’s credit account information. Obviously, this information is not for any saintly purpose. He may use the information to transfer the company money to his own personal accounts!