Every quest needs the wise old mentor who guides the hero on his way. In this story, that role is played by Grid. Grid's a giantess, which tells us that not all giants are cut from the same cloth. Plus, that puts her in-the-know about who's who in the giant world: She knows that Geirrod is not to be trusted, and she's happy to share that information with Thor. Why? Well, she might be helpful out of the goodness of her heart. But she's also got family ties to the thunder god. She's Odin's mistress and the mother of Thor's stepbrothers, so her eagerness to help Thor could come from an abundance of family feeling.
That family feeling is something Grid has that her foil, or contrasting character, Geirrod, doesn't. Grid's presence makes this lack in Geirrod clear by contrast. Grid's great hospitality to Thor also contrasts with the way Geirrod treats him.
We've said it once and we'll say it again: Grid's the yin to Geirrod's yang.