When might engineers use a topagraphical map

To locate interesting landforms, to find their way through an area,

Explanation:

Generally a topographic map is used to share information of certain area, it describes the distances between the points on the surface of the earth and their elevation levels with the help of contour lines.

One can understand fully an area with the help of a topographic map, even without visiting that area.

So a topographic map gives you three dimensional information of an area.

It is mainly required before the design of any thing over any area.