When an organisation needs to report on its supply chain emissions, one company might have dozens, hundreds or even thousands of suppliers, so getting specific information from all of them is problematic. It is also difficult when some suppliers give you one kind of data and others give you a completely different type, because that makes it very hard to run an assessment on a like-for-like basis.
A spend-based metric gives you an intensity metric for every pound spent on a typical or specific type of product or service. If you are spending a known amount on a particular product line, you can apply that metric to understand what the footprint would typically look like at that level of spend. Applied across the board it gives you consistency, which is why it is often a good way for an organisation to start supply chain reporting - it at least establishes what the emissions are likely to be for the spend you have.