The main problem SDKs solve is providing a way to interact with an API without having to worry about nitty gritty details like serialization and deserialization, authentication, error handling, retries, connection and thread pooling, etc.

When you have a good SDK, interacting with an API feels like working with regular functions and objects in your chosen programming languages, so it _greatly_ simplifies integrating with an API.

Stainless's Series A announcement post might be a good one to read to get the gist: https://www.stainless.com/blog/stainless-series-a