This action gives mobility to your units.
But it causes fatigue and his cost differs with unit type.
If a road is built on a cell your unit occupy, movement cost is divided by 2. (Great buddy engineer !)
An unit can't move on a enemy held fortification.
Only a few unit types can hold fortification.
A river can't be crossed with a motorized unit, except if a bridge is built in the cell. (Very cool buddy engineer!).
So, cool buddy engineers can cross rivers and build bridges...