Isn't it pretty clear why they do this? It's just another test to weed out the weak so that the people they invest in stick the course and are successful.

Read any pro's bio - it's clear that becoming a top pro wasn't a bundle of laughs and took plenty of mental tenacity, e.g. Cavendish was told by the UK coaching team: "We decided we didn't want you on the team any more, go ride for this continental bunch in Germany you've never heard of and see how you get on. What, you don't speak German and don't like the food? Here's the ticket. Suck it up...". Imagine what would happen if your current employer did that to you? Brutal is how it is.

Again - if it wasn't helpful, then the coaches wouldn't do it. Their jobs are on the line each year and there is no place to hide if their riders don't perform so they need to try new things and keep re-inventing what they do to stay ahead of the Brits (or catch up depending on your PoV).