Unless you put all your money into an income annuity, you won't be able to cover expenses with no money in your portfolio.

If you do solely "invest" in annuities, you'll have income that may be enough to live on, but will leave nothing to your heirs (other than, possibly, your home). You may be ok with this, but not everyone feels the same.

In our case, we will almost surely leave a healthy sum to our kids because that money will be what generates the income to cover our retirement expenses including travel and other experiences. To avoid running out while we're still alive means there will be lots left over, which I'm fine with.