In short lower costs and better diversification but let me copy & past the answer given by the famous boglehead Taylor Larimore in this post .
Usually lower Fund/ETF Expense Ratios: Total Stock Market (VTSAX & VT) = .05%; Total International (VTIAX & VXUS) = .11%; Total World (VTWSX = .21%; VT = 0.11%).
Lower Turnover (hidden cost): Total Stock Market = 4.0%; Total International = 3.1%; Total World = 14.8%
Tax Efficiency (5 years): Total Stock Market =.56; Total International = 1.09; Total World = .82
Better diversification (lower risk): Total U.S. Stock Market and Total International (combined) hold 9,773 stocks. Total World holds 7,774.
More U.S. stocks: Total World contains approximately 53% U.S. stocks. Many authorities, including Mr. Bogle, believe this is inadequate for U.S. investors.
Flexibility: The ratio between U.S. Total Stock Market and Total International is flexible for investors wherever they live in the world or whatever their desire.
Admiral Shares: Unlike Total Stock Market and Total International, Total World has no Admiral shares.