Some people like the contrast of the materials with the greenery. Contrasts can add interest. Imagine a shady bed of hosta by itself, and then add an urn, with impatiens or trailing ivy. It just got better. Some items actually work, and that's their value.
Troughs provide water for birds and pets.
Hewn stones and antique splashblocks look a lot better taking the water away from the house than the hard rubber ones do. Some people like the different sounds of water a fountain makes. On top of all this, these antiques are very sculpturally pleasing. I've seen gardens with large 19th century pottery ollas tucked in nooks. Each olla had a different color and shape.