Because it's often an excellent experience and makes the vacation better. I rented out a bedroom in my 2BR apartment in NYC for several years and it was truly a wonderful experience. I had a one-week minimum and therefore hosted almost entirely tourists visiting NYC from overseas (as opposed to random people in NYC who needed a place to crash for a night).

With very very few exceptions, I met warm friendly people. They liked have a local in the same place to talk about plans and get suggestions, I liked some new faces to have coffee with in the morning. Everyone won.