In a taxable account, net long-term capital gains are taxed at a rate lower than the ordinary income-tax rate for withdrawals from tax-deferred retirement plans.
Selling out of losing investments inside of a taxable account allows you to realize both short-term and long-term capital losses, which you can use to offset gains for tax purposes.
Long-term capital gains and qualified dividends are taxed at lower rates than interest earned on a bank account (if any these days) or wages from a job.