Bond prices move inversely to interest rates: interest rates up, bond prices down.
With interest rates near zero (although not likely to go up anytime soon) there is in theory only downside risk to bonds.
However, bonds, especially .gov issued bonds are considered zero principle risk and thus are used to park funds in a zero/low risk vehicle.
Traditionally, bonds were used as a source of "guaranteed" income but with historically low interest rates so low for so long, they haven't been a good vehicle for that but rather a way to reduce overall portfolio volatility.
Bond prices tend to move inversely to stock prices as well: in a zero sum game, the $ just sloshes back and forth between equities, bonds, commodities, etc. So while bonds generally reduce overall portfolio return, there are times (like now) when they a) reduce portfolio volatility and b) reduce losses.
Backtesting has shown that an optimal (i.e. higher than without any bonds and with reduced volatility) return can be had with around an 80-20 to 90-10 split between equities and bonds.
That said, the most $ I ever made in a relatively short period of time was trading bonds back in the 80s (when the 30-year hit over 15%!); I was satisfied with a guaranteed returns of 12, 13, 14 and 15% for 30 years...however, the real play was when those interest rates went back to nominal over the next 20 years...and I've been waiting ever since for another opportunity like that although the economic conditions which warrant a coupon of 15% have their own perils.