The electron beam can make as many as five trips around the tunnel. This saves money since the finished machine can be built on a smaller plot of land and, more importantly, the beam can pass through each one of the expensive acceleration cavities more than once.
Doing this creates a problem. All of the electrons coming from the South Linear Accelerator are mixed together in one pipe. Some electrons have been around the accelerator more than others. The more trips an electron has made around the accelerator, the more energy it has and the less it bends when it passes by a magnet. This makes it impossible to keep the electrons together in one pipe while steering them around a corner.
To make the job of steering possible, the electrons are sorted by energy. When the beam passes through the large blue electromagnet the electrons that are on their first trip around the accelerator are bent a lot and end up near the ceiling. Those that are on their fifth pass barely notice that the magnet is there and stay near the floor. Now each pipe has electrons with only one energy, so they will all bend the same when they pass by a magnet!