While capturing portraits in the studio can be great for optimal control and convenience, portrait on location can often say so much more about the sitter (the person in the photo).
Photographers who take portraits on location, have limited control of the environment. Any lighting that's used for the portrait also has to take into account the existing light already there.
On the plus side, the environment surrounding the sitter so help complete the story about them, or create intrigue. We also find the some sitter feel more comfortable shooting out of the studio in a familiar place. It can help them come out, feel natural and interact with what's around them. The studio can sometime feel barren, when shooting on location there's an element of surprise that often adds interest to the photos.