When you buy flowers from a florist or a good grocery floral department, most of the foliage has already been stripped from the stems. Roses arrive with leaves all the way up, but they leave the shop with only a few at the top. Lilies come in with a leaf at every node, and half of them are gone before they go into a bouquet. It looks like a cosmetic choice or a way to show off the blooms, but the reasons are more practical than that.
Foliage below the waterline rots, and it rots fast. Leaves stuck underwater start breaking down within a day, and the bacteria that decomposition releases is what kills flowers. It clouds the water, blocks the stem ends, and turns a vase into something that needs scrubbing. Stripping everything that will sit in water is not optional if you want the flowers to last.
That part is obvious to anyone who has seen what happens when you skip it. What is less obvious is why florists strip well above the waterline too, sometimes taking off two thirds of what the grower left on.