No one wants to take unnecessary risk with their money. Investment risk basically means you can't be sure what the results of your investing will be. Inflation, however, represents a different kind of risk.
Inflation means that every year, the same amount of money buys less.
Inflation isn't much of a problem for money you're going to be using soon. You won't lose much buying power over a year or so, and it's probably more important to preserve what you've got. But the longer inflation has a chance to nibble away at your money, the more inflation risk you run. Most investments generally involve more of one type and less of the other. To reduce one type of risk, you have to take more of the other.