Herbaceous pruning (cutting the foliage of perennial plants) has a few uses. Often, it is used to delay the bloom of a plant, or to make it have better branching instead of just one main stem. This is known across the pond as the "Chelsea Chop", because gardeners in the UK can plan on cutting their perennials around time of the Chelsea Flower Show. (The show is usually held at the end of May.)
Another good reason for pruning perennial foliage occurs later in the season. During the hot months of July and August, perennial foliage often starts to look old and tattered. For many years, I sighed and assumed that yellowing leaves meant my garden was passing into its fall foliage already.