Big plants go to the rear of the bed generally and the smaller ones go closer to the front. This insures that the flowers can be seen. There are exceptions to the rule, but the basic principle works. Some plants have big wide leaves and some have spiked ones. Some plants have crinkled leaves and some are smooth.
Some flowers are trumpet-like and some are more full.
Some flowers are held high above the foliage and some are placed low. The variety continues on. It's nice to mix things up a bit and put in plants that have a variety of looks to them in the same bed so that things don't get boring. The differences in the plants show up best next to something that is different. This creates more interest in the garden.