As educational technologies become increasingly important in K-12 physics education, it is important to understand why and how teachers choose to adopt certain technologies. We examined 2000 responses from a survey of high school teachers on how they used PhET interactive simulations (mostly in physics) and what value they felt it provided their students. The analysis helps inform what aspects of an educational technology support or hinder its adoption. First, the teachers valued flexibility. They used simulations in multiple ways for a variety of learning goals, primarily: understanding content, science process, and motivation. Second, they often chose implementations in which students control the simulation. Finally, they noted the value of specific affordances: visualization, seeing and controlling what's normally not possible, and manipulation. © American Association of Physics Teachers.