I remember my first experience with .Net Framework and C# after years of MFC/COM/DCOM torture with lovely C++. Whoa, I don't have to track every reference and resource allocation? That GC thing rules! No more memory leaks! Right... Let's say, you're tasked with integrating Sitecore Commerce 9 and some PIM. It totally makes sense to use the Sitecore Commerce catalog system, so your integration would imply importing catalog structure and contents from PIM. While API capabilities of PIMs may vary, pretty much every type of PIM supports file exports of catalog. The native Sitecore Commerce catalog import/export is still in its infancy (no support for partial, incremental, or selective operations, not to mention lack of converters into its pretty unorthodox format, as well as inability to import product assets), so your only option is to build a custom catalog import plugin. The code for creating catalogs, categories and sellable items is relatively well-known and somewhat