There is a purpose: it makes life much easier for the camera operators and graphics people. Just about everything produced these days is produced in 16:9 widescreen. If your broadcast is going to be "center-cut" for presentation to viewers using 4:3 displays, then every camera shot has to be set up to be "center-cut-safe": you can't put anything important in the wings on either side of the 16:9 image that will be cut off on the SD 4:3 feed. What's more, every graphic has to be "center-cut-safe," too. The result is both aggravation for the camera crews and producers and space that ends up being wasted on the "wings" for viewers watching 16:9 screens, which is an increasing number of viewers.
So they simply produce everything in 16:9 and letterbox the 4:3 SD feed. There will be more and more of that in the years to come, as 4:3 production is now nearly obsolete. When was the last time you saw a 4:3 set for sale in a store?