I represent a truck driver.  What is a manifest and when is it needed
by a truck driver?

Here is a definition of "manifest appropriate to the trucking world:

"A shipping document for any of several forms of transport which lists
origin, contents, value, destination, carrier, and other pertinent
information for use at terminals or a custom house."
www.crfonline.org/orc/glossary/m.html

The manifest documents what has been loaded in the truck or
semitrailer/container, as this exerpt from another site explains:

"Manifest
A manifest is an itemized list of the contents of the shipment, to be
shown to officials for customs clearance. Another name for the
manifest is cargo control document (CCD). The most commonly used
manifest is a Form A8A.

The carrier prepares the manifest based on the information provided by
the shipper. The carrier must provide the customs broker with a
manifest in order for the broker to obtain a release from Customs.

A manifest or CCD has its own identifier, called the cargo control
number. Once submitted and accepted by Customs, the manifest and cargo
control number are monitored by Customs to ensure the proper clearance
and closure of the shipment."
http://www.livingstonintl.com/shippingtocdab.cfm?name=Documentation&pageid=1269&action=published

Disregarding that this exerpt mentions customs, the point is that the
manifest is based on the shipper's information. The carrier/driver
just controls that the shipper's items are received and then delivered
and receipted.
The manifest is is the control document for the shipper and the carrier.

This is a free comment, not an official answer to your question.
Someone may wish to correct or amend.
If you have any question, please let me know.

In addition to what is already posted, I'd like to add that a manifest
shows the total shipments on a trailer, the total gross weight, and
any/all HAZMAT onboard. The manifest should also reflect the origin of
loading, the destination, the time of loading, who loaded it, and how
many units were loaded.