Once a bond is issued, it becomes a tradable commodity. Its value on the open market will rise and fall as interest rates change or as faith in the issuer changes. But when it reaches its maturity date, any interim variations in price are of no consequence because the bondholder receives the Par value assuming that the issuer is capable of repaying the loan. Note that a bond is a debt obligation, and does not represent ownership. Bonds provide a known rate of return Thus bonds provide a steady, known income.
Bonds return a known value on their maturity date.
This provides security for your money provided you hold the bond to maturity. Bond yields are generally superior to money market funds, certificates of deposit, and bank accounts. Yields are related to liquidity and risk. Since bonds vary in price during their life, you could take a loss if you unexpectedly had to sell a bond prior to maturity. Also, the issuing company could go bankrupt and be unable to repay the loan.