There are two reasons. First, if you wait until it is a mandatory requirement you are putting yourself behind the curve - starting now demonstrates that you are choosing to do this rather than being forced into it by legislation.
Second, you are able to offer more value to your clients. Every single business is part of somebody else’s supply chain, so capturing your carbon footprint and understanding where those emissions come from does not only assist your own journey to net zero. It also helps anyone who buys from you, because you can demonstrate the impact of their spend with you - which is something your customers are going to be required to provide more and more. Doing it now means you get ahead of the curve, you differentiate your business from your competitors, and your clients achieve something that helps their own journey simply by buying from you.