When you invest in a bond mutual fund, you have the option of choosing how the income and capital gains will be reinvested. Capital gains generally occur once per year, and should be generally harmless to simply reinvest in the same bond fund. Income distribution generally comes monthly, and I feel should be directed into a money market fund instead.
The main reason is less headache at tax time. If you reinvested income in the same bond, you will have small monthly purchases all the time. This can be a chore when you are trying to calculate the basis of a bond sold. Purchasing each month will also guarantee the triggering of the so called 'wash sale' rule when selling some bond. When you have 'wash sale', the basis calculation is further complicated.