Some things are so obvious as to not need an explanation... choosing perennials for the garden based on their flowers and/or their foliage is one of them. When it comes to ferns and other plants like Acanthus mollis, however, there is an additional reason. I love to see them "emerge" in the spring. Seeing the leaves just as they stand above the crown of the plant and as they "unfold" is fascinating in itself.
Not to say the blooms aren't interesting too.
Now, if I can just get my hands on the variegated form: