Why, or why not?
Note that I do not necessarily mean rolling a die and consulting a table during game play, but rather "filling in the blanks" or "fleshing out" a particular stretch of the game.
For example, I'm running a published minicampaign module in a published setting in which the party must traverse a vast expanses of forest, and a little voice in my head says, "Something should go here." So I tossed in a planned-but-unrelated encounter with some giant spiders.
When I was running it, it felt really random, and I was worried that my players might think I was wasting their time, but as one of my players said to me later, "Sometimes, when you walk into Mirkwood, you get attacked by spiders." His point was that random encounters are a good thing, as they remind the players that the world is larger than whatever quest or journey they're on.
Plus it gave the herbalist a chance to use her healing abilities.