Sending some types of data (for example, a streaming video) across a network, as one complete communication stream, could use all of the available bandwidth and prevent other communications from occurring at the same time. It also makes error recovery and retransmission of damaged data difficult.
The figure shows that segmenting the data into smaller chunks enables many different communications, from many different users, to be interleaved (multiplexed) on the same network. Segmentation of the data by transport layer protocols also provides the means to both send and receive data when running multiple applications concurrently on a computer.
Without segmentation, only one application would be able to receive data. For example, a streaming video, the media would be completely consumed by the one communication stream instead of shared. You could not receive emails, chat on instant messenger, or view web pages while also viewing the video.
To identify each segment of data, the transport layer adds to the segment a header containing binary data. This header contains fields of bits. It is the values in these fields that enable different transport layer protocols to perform different functions in managing data communication.