Investors nearing retirement also must bear in mind how much they can afford to risk as the time to draw down the money nears. This question is primarily a time management question. If stocks, and bonds for that matter, only went up in value, then there would be no question to answer. You would merely put your money in the highest-returning asset and let it ride. As a person nears retirement, bonds become more interesting.
Bonds do fluctuate in value because interest rates swing with inflationary expectations.
Consequently, many investors believe that bonds carry the same risk as stocks. But this isn't the case, particularly when bonds are held to maturity. You can always hold a depressed bond to maturity if you can't bear to sell it for a loss. With a stock, you can only pray that it rebounds before you need to sell. And, of course, when you buy a common stock, no one is obligated to ever return your principal to you - ever.