per @epriestley:
Possible Use Case: Alternate Short Domain
We could let users configure a short domain which pipes directly into Phurl. For example, an install might be hosted at phabricator.mycompany.com, but a shorter domain like mcmp.me is configured as the Phurl domain. Phurl then recognizes requests on this domain, and emits URLs using this domain (https://mcmp.me/2). The old URLs still work correctly.