A bond mutual fund has within it multiple bonds, and for that reason it is impossible to lock in the payment rate or the principal, which you would be able to do if you were directly buying a fund.
A bond mutual fund is an investment company which manages a portfolio of individual bonds. The investors buy ownership in the company, and each share represents ownership in all of the company's holdings. Managers will use these investments to buy and sell bonds that align with the objective of the fund.
Because a bond fund manager has more resources to deal with, they can invest in a vast array of bonds - many more than could any individual investor. There are also certain investments that cost tens of thousands of dollars a share - a bond fund costs far less.
Liquidity plays a major role in bond buying. If you purchase a bond individually and wish to sell it, you must find a buyer for your bond, but if you are invested in a bond fund, that fund has to buy your shares back at any time you wish.