You're not buying the points in the usual sense.
If I had 40,000 points and wanted to stay in IC London, I have a few options:
1. Pay cash at something like £170 per night (don't correct me on the price, it's just an example). Leaving me with 40,000 balance.
2. Use all 40,000 points for a reward night leaving me with zero balance.
3. Use 30,000 points and pay $60 instead of the other 10,000 points. Leaving me with 10,000 points towards another night somewhere.
In non of these situations will I actually receive miles from IHG. Rather than buy 10,000 points for 1.1 cent each and use them with the 30,000 in option 3, I'm paying 0.6 cents each for them to add the on my behalf.
Think this makes sense, difficult to re-read as posting from my iPhone.