Glean is focused on storing and querying data about the code. The idea is that you have your own program to collect that data, then you use Glean to store that compactly and to have snappy queries.

You would create entries like "this is a declaration of X", "this is a use of X". Then you can query things like "give me all uses of X" in sub-millisecond time. You hook that up to an LSP server then you get almost zero-cost find-references, jump-to-definition, etc. The snappy queries also mean it becomes possible to perform whole codebase (and cross-language) analysis. That is, answering questions like "what code is not referenced from this root?", "does this Haskell function use anything that calls malloc?" (analysis through the ffi barrier).

One can also attach all kinds of information from different sources to code entities, not only things derived from the source itself. You add things like run-time costs, frequency of use, common errors, etc, and an LSP server could make all of it available right in your editor.

For very large or complex codebases, where it is just too expensive or too complicated to calculate this information locally a system like this becomes very useful.