For the same reasons that someone would prefer a 1-year, 5-year, or 30-year regular bond. Longer time to maturity means higher yields but more interest rate risk, so NAVs will be more volatile. Some may know the timing of their future obligations so they will match the duration of the bonds to coincide.
I believe the total bond market fund holds a variety of durations (not sure about the mix of credit ratings but you could check on Yahoo finance) as well as different types of bonds, like government, corporate, and mortgage-backed.