AWS applies the benefits of RI and SP commitments to your usage on an hourly basis. If there isn’t enough eligible usage, the commitment will cost you more than it saved for that hour. Individual environments, managed by individul teams, may not have enough eligible usage to make a commitment cost effective so miss out on savings. We avoid this by committing based on your combined usage.
RIs and SPs combine together in a way that makes it easy for one to displace another and accidentally put you into a position of wasting money. A central commitment management team avoids this conflict.