Test cases are useful while writing a traceability matrix code example

Example: forward traceability matrix vs backward traceability matrix

Mapping test cases to requirements are
called Backward Traceability Matrix.
It is used to ensure whether the current
product remains on the right track.
It makes sure that we are not expanding 
the scope of the project by adding 
functionality that is not specified
in the requirements.

Mapping requirements to test cases is 
called Forward Traceability Matrix. 
It is used to ensure whether the project
progresses in the desired direction. 
It makes sure that each requirement 
is tested thoroughly.