The shape, the softness, the volume all say femininity in a way that fosters both comfort and arousal - even though they are visible in how they shape clothing and potentially as cleavage, they are off limits to all people most of the time, and most people all of the time, shared visually and physically only in a limited context. The way in their pliability they move about when uncovered draws people's notice of movement.
They are just an indication of something enjoyable to explore, even on a spouse of decades, because it is being permitted into such a personal space with the lady sporting them. This is the reason sucking boobs is the most favourite thing of woman and man loves it. It gives pleasure to both the partners.