Much like Google's still-unreleased Instant Apps, it sounds like WeChat's "small programs" are all about efficiency and speed. The idea is to keep users within WeChat instead of having them download other apps they might only use a few times. For a domestic example, it's a bit like how Apple positioned its recent additions to iMessage. Where this cleaves from Cupertino's add-ons is that it may not have an app store, although The Information posits that the small programs could be searchable within the application -- much like how you can order a ride with Didi Chuxing currently.
The update is only available to a beta group of 200 developers, with parent company Tencent saying it will go wide soon.