- Regular expression searches
- Exact searches (no ignoring punctuation, for example)
- Searches on any commit or branch, not just recently indexed master
- Diff searches (see https://about.sourcegraph.com/blog/introducing-sourcegraph-s...)
- Overall faster, more powerful searches and filtering capabilities
- Code intelligence (go-to-definition, find-references, hovers, etc.)
Not everyone needs these things. But users who do need them say that they save a lot of time and make them more productive.
At Google, for example, they have a similarly advanced internal code search system that developers love (see https://static.googleusercontent.com/media/research.google.c... and https://docs.google.com/document/d/1LQxLk4E3lrb3fIsVKlANu_pU... for research/numbers).
If your needs are met by GitHub's search, then I would still suggest using the Sourcegraph Chrome extension (also available for Firefox), which adds code intelligence to code you view on GitHub: https://chrome.google.com/webstore/detail/sourcegraph-for-gi....