Many things. Capacity, Bandwidth, Serviceability [1]. It also paves the road towards disaggregated memory [2].

edit: The official Compute Express Link (CXL) website has an article about the benefits of this technology [3].

[1] - https://www.nextplatform.com/2021/09/07/the-cxl-roadmap-open...

[2] - https://pmem.io/blog/2022/01/disaggregated-memory-in-pursuit...

[3] - https://www.computeexpresslink.org/post/the-benefits-of-seri...