We, as a nation and as a people, are vulnerable to so very many outside threats that clearly, the only logical solution is to build a fleet of giant mechanical war machines of varying sizes to protect ourselves.
Threats giant robots can deter include (but are not limited to):
Terrorists
Aliens
Nazi vampires
Twilight sequel/prequel/spinoffs
Russia
North Korea
Uwe Boll movies
Giant monsters
Shao Kahn
Bon Jovi
Big-ass spiders
Jar Jar Binks
The creation of these machines will also spur job creation to design, build, and maintain them, in addition to the necessary constructions jobs that will occur from the tragically (and hilariously) unavoidable collateral damage.
Some may ask how we can afford to spend ludicrous amounts of money on giant robots. To them, I counter with, "how can we NOT?"