To replace the silicate bag with something a) new and b) more robust. It is not designed for our water bottles but, by coincidence, clips in where the bag used to sit. It's an option if you want to remove the bag but want to avoid the inevitable coolant change required if you do as existing coolant will age quicker without the silicates.
See here:
https://www.vwroc.com/forums/topic/38040-coolant-bottle/page/3/PS, when I say new, I am referring to it being in better nick (and therefore less likely to fail) than your 10 year old bag, not that it contains anything new or fancy,