Backends can change or extend how a Treehouse application behaves and are exposed to the frontend via adapters.
The Treehouse frontend uses its backend adapter to store the state of your workspace into a JSON document. The backend can decide where and how that JSON document is stored. For example, the Browser backend adapter will store the JSON into localStorage.
An optional capability of the frontend is to know whether a particular user is authenticated. You typically want user authentication for cloud or web-based deployments, but not necessarily for a local desktop app.
Out of the box, Treehouse will index your workspace for full-text search using Minisearch, which will be good enough for many cases. However, a backend adapter can choose to hook into the index and searching so that you could have a more powerful search index such as ElasticSearch.