Drivers who drive liveried trucks, often put their names in the window, so that when their mates come the other way, they know who is driving the truck. Some drivers just do it for show, like car drivers used to have their names on sun strips.
The idea of putting CD's in the window was started by someone who gave out the rumour that the 'shiny' side of a CD would reflect the flash from a speed camera, thus blinding it. Unfortunately this doesn't work. So, either some people are misinformed, or some have decided to leave them there for the hell of it. Regarding blocking the view; it is part of the HGV MOT that you must have nothing blocking the wiper swept area of the windscreen. So when all these trucks with flags etc go for test, they all have to have their 'adornments' taken down first. On my truck I had a mobile phone aerial that doubled as a tax disc holder. It was just in the swept area in the bottom left of the screen. The examiner said as it wasn't being used as a disc holder, it had to be removed... and the truck had passed its test no problem the previous 2 years! Just depends what their little bugbear is!