The myth of Thor and the Jotun Geirrod follows a quest structure. That means that the hero leaves his home, the place where he feels safest and most comfortable, to journey to a scary, foreign place. Like most quest stories, a lot of this one takes place on the road. That's where you'd expect to encounter danger in a story like this, right? And sure enough, Thor and Loki meet with a giant waterfall of pee, unleashed by Geirrod's daughter, that threatens to drown them as they're crossing the River Vímur.
Thor's crossing of the river symbolizes his passage from his safe, comfortable world to one where everything is the opposite of how it should be. (For more on that, see "Symbols.") It doesn't take long for Thor to tire of this topsy-turvy world and crush all the giants to a pulp. When he does that, he's trying to set the world right again, to turn the mead hall into the safe space it should be.
Usually, the end of a quest story tells about the hero's return to his safe, orderly place. But by killing all the frost-giants, Thor's already restored order. (Although if you're a giant, you might disagree.) Maybe that's why this story ends when the giants die, and not when Thor returns to Asgard.