"One reason is that the truly wealthy employ all kinds of legal means to minimize their tax liability, including shifting income around the world, deferring gains on their assets and many other sophisticated strategies. Another, though, is that taxes on ordinary income simply don’t apply to inheritance or investment, principal sources of wealth...But what is being taxed is often just a small portion of the income and wealth of the very richest Americans; unearned income, including unrealized gains and gains on investments, is either not taxed or taxed at a fraction of the top rate on wages...Taxing wealth in addition to income is one way to make sure that the rich contribute more to government coffers. That would essentially be a tax on household assets like property, stocks, bonds, unincorporated businesses, trusts, art and yachts...The idea is to aim at the wealthiest part of the population, perhaps the top 1 percent, a group that has seen the most significant and consistent accumulation of wealth over the last few decades...Here is how that can work in practice: A billionaire can borrow against his stocks, art and real estate, and spend that borrowed money without paying tax. All he has to do is pay interest on the loan. When he dies, his heirs can sell the assets to pay off the debt. Under an existing rule known as the “step-up in basis,” no matter how much the assets have appreciated in value, no one will owe income tax on that gain. And the rest of his fortune goes to his heirs without anyone ever paying income taxes on the appreciation in the assets."(NYTimes explains it well)