In Harvest Moon: Sunshine Islands, you had to speak to villagers daily. If not, your friend level would decrease, and you would get reprimanded. Building relationships between villagers is of high importance, but little will come from it. In Harvest Moon: Friends of Mineral Town, you may receive a few extra items, but nothing after that.
Adding more friend interactions would increase the immersion of each game. In Story of Seasons: Trio of Towns, Komari and Carrie excitedly tell you how much they adore you as a friend. But besides a few friendship events and festivals -- or perhaps sharing a meal with them -- you rarely interact with either character, which kind of makes the friendships moot at a certain point.
Except when being forced to befriend someone for a service or item, friendships currently don’t add much to the game. Adding more events and interactive sequences could make friendships more meaningful.