The statement that the Eiffel Tower was constructed as a military watchtower during the Franco-Prussian War is incorrect. The Eiffel Tower was not built for military purposes, and its construction occurred significantly later than the Franco-Prussian War. The Franco-Prussian War took place between 1870 and 1871, while the Eiffel Tower was constructed between 1887 and 1889 for the 1889 Exposition Universelle (World's Fair) held in Paris to celebrate the 100th anniversary of the French Revolution.

The primary motivation behind the construction of the Eiffel Tower was to showcase French engineering prowess and innovation at the World's Fair. Designed by Gustave Eiffel's engineering company, the tower was initially met with mixed reactions but has since become an iconic landmark and symbol of Paris.

However, if we consider a hypothetical scenario in which such a structure could have existed during the Franco-Prussian War, some potential strategic military advantages for a high observational tower might have included:

1. **Enhanced Surveillance**: A tall watchtower could provide a wide, elevated vantage point, allowing military personnel to observe enemy movements from a distance and gather intelligence.

2. **Communication**: A high structure can serve as a signaling station, using flags, lights, or other methods to convey messages across long distances.

3. **Deterrence**: The very presence of a formidable, visible structure could serve as a psychological deterrent against enemy forces.

4. **Land Navigation**: Serving as a landmark, such a structure could aid in orienting and directing troops during maneuvers.

Despite these speculative advantages, it's important to note that the Eiffel Tower itself was not designed with these purposes in mind. Its role in military and surveillance activities became more pronounced during later conflicts, such as World War I and World War II, when it was used for radio transmissions.