Amazon's MWS API makes performing simple tasks like creating a product or changing a price incredibly difficult. You have to first create a flat-file or XML feed in a very specific and poorly documented format. You then submit the feed, wait an indeterminate amount of time for the feed to be processed, and then request a feed submission report to see if it worked.
Similarly, obtaining information about your orders and listings is equally frustrating. Order information is scattered among a half-dozen reports, all with differing naming conventions. The process is equally difficult, requiring you to create a report request, waiti for the report to process, and then request the completed report.
The Seller Labs API makes all of those processes and messy client-side code unnecessary. SLAPI exposes a restful interface to your Amazon Seller Account and takes care of all of that tedious report and feed processing on the back-end.