A spend-based metric is very much a benchmark. It tells you the emissions associated with that level of spend to an industry standard, as opposed to the specific emissions of your own supply chain.
When you want to progress, you start to work much more in collaboration with your suppliers to get specific information from them, so you understand exactly what their carbon footprint is and exactly how it impacts you based on how much you spend with them. That is always more detailed and more accurate, but it can be difficult to achieve straight away because of the volume of suppliers you might have, or the complexity of what has to happen to collect it. So a spend-based metric is generally a good starting point, and working much more closely with those suppliers to get their actual emissions is how you improve the quality of the data and the credibility of the reporting.