I don't know specifically why it was done where you saw it, but I have seen that done before when some processing has been done on the values in one array or another and you want to merge those changes back into $_REQUEST so that anyone using $_REQUEST will get the changes even though they were done to the $_POST or $_GET variables.
This comes up in situations like Wordpress has because plugin developers could be using any of those variables to access data and the Wordpress core would need to make sure they all get the same data.
Why wouldn't you want to do it to $_REQUEST directly? Because $_REQUEST contains a ton of extra info that $_POST and $_GET don't have. You might not want to apply your processing to all those extra bits.