Don't know if you've been up there or not, but I have and it's about as exciting as it sounds. I wondered this as well but couldn't determine any obvious use for it. It was just a big copper eye-sore in an otherwise incredible landscape. As there has been a scientific station up there for a very long time, it may have been thought irrelevant in the days when there were few tourists. If it's anything like England, it's probably a bloody listed building nowadays! The views from up there are awesome, I stayed until I was almost frozen and then walked back to Chamonix from the mid-way station. If anyone plans to do this, wear good boots (unlike me!). Also, the temperature at the top is very low even in mid-summer and can be a shock.