It was added to the 1099-DIV this year in order to aid in the calculation of Section 199A, also known as the Qualified Business Income Deduction.
According to the IRS:
"New box 5 section 199A dividends. Box 5, section 199A dividends, must be completed to report section 199A dividends paid to the recipient. The amount paid is also included in box 1a."
https://www.irs.gov/pub/irs-prior/i1099div--2018.pdf
