Arsenal is one of the biggest clubs on the face of the Earth. It has a massive following here in the United States as well. Exploria Stadium will almost certainly be sold out, or at least approaching capacity, and hundreds of thousands of fans across the globe will be interested to some degree.
High Level Competition
Playing good teams is really good! It helps test the group against the best of the best and can be a fantastic opportunity for young players. Even if it's a friendly, it's still competition against one of the most talented squads on the planet, and that can be a valuable experience.