Pavilions typically don't add value to a home because they aren't attached to the home and thus don't count toward the house's total square footage of livable space. However, a pavilion can add curb appeal when well-built in a neighborhood where people enjoy being outside most of the year.
Keep in mind that even in cases where a pavilion does attach to the home roofline with a gable-style top, it still won't add to square footage because it isn't enclosed.