"PostgreSQL provides various lock modes to control concurrent access to data in tables. Advisory locks provide a convenient way to obtain a lock from PostgreSQL that is completely application enforced, and will not block writes to the table."
This is not like row-locking your tables, this is to help you ensure concurrency for certain parts of your application. eg that you only enter a complex, resource-intensive part of your codebase once across all your servers. Given that ruby basically isn't very threadsafe - this provides a mechanism for concurrency-locking that is available across your plausibly widely-distributed servers by basically storing the semaphore in the mutually-accessible database.