Interchangeable sunglasses can be very useful in situations where space and weight capacity is limited, such as backpacking trips or long bike rides. They allow you to respond to changing conditions while taking up a minimal amount of precious space. Look for semi-frameless designs that shave weight and make lens swaps quick.
Sometimes you just need to change your look up a little. Maybe you want to rock reflective lenses to stand out in the crowd, or perhaps you want to go with black lenses for a more understated look. Whatever the reason, interchangeable sunglasses allow you to mix things up.
Lens tint selection comes down to more than just looks. Darker tints and mirrored lenses allow in less light than lighter tints, and different color lenses affect color perception and contrast as well. Polarized replacement lenses reduce glare from reflective surfaces like water and snow.