All it does is transfer money to the contractor doing the digging (in America's case, the military-industrial complex) while everyone else becomes poorer.

$8 trillion has been spent on the War on Terror so far. Like I said in another comment, that's enough money to build 80 million $100k homes, or reduce America's debt by 25%, or pay off all student loans, or build 400,000 KM of high-speed rail at $20M/km, or give every American taxpayer a one-time check of $48k, etc.

Every bomb dropped on Afghanistan or Iraq was money diverted from something else useful the US could have done.