[Zimahl]

The government doesn't want it's citizens to horde money. A ton of money sitting in the bank or in assets doesn't really do anyone any good. An inheritance tax pushes the rich into using their money - instead of leaving your kids cash and assets, you leave your kids a company or something similar. They can either continue to own the company and get paid for it or sell it and get taxed on it. Regardless, a company that provides jobs is a much more interesting prospect to the gov't than just rich people with assets.

[RhysU]

A metric ton of money sitting in a bank is invested when the bank makes loans.