Also known as croys in Scotland, and cribs on the Wye.
As Bert says, in many cases they aren't actually intended as casting platforms, but as a way of directing or manipulating the current to create better holding lies. Sometimes they are designed create an area of slower water in a fast run, but they can also speed up the flow in slower pools, and often the water will scour out a nice deep hole just off the end of them.
On the whole they are built for the convenience of the fish, not the fishermen, and it's usually worth spending time fishing the water immediately around them. Don't just think of them as casting platforms - this can encourage people to march straight out to the end of them, scaring the fish from lies under their feet while casting as far as they can into less productive water in the middle of the river.