Under what circumstances would the transport layer use segmentation?

The transport layer segments data whenever the size of the packet is too large to transport efficiently--in practice, this happens whenever the data is too large to fit in a single network layer packet. Although the network layer can also fragment packets, the process is inefficient and generally the transport layer segment size is adjusted so that no further division is required in the network layer.

Much more details are here:

http://en.wikipedia.org/wiki/TCP_header#Maximum_segment_size