Backend development tasks often divide cleanly into two categories: well-specified implementation work (add an endpoint, extend a schema, wire up a new service) and design work (decide how to structure the data model, choose the right abstraction, handle the edge cases no one thought of). The first category consumes most of a senior engineer's time even though it does not require their judgment.
The bottleneck is that a senior engineer is required for both categories. Agents cannot replace judgment on system design, but they can execute well-specified implementation tasks, freeing senior engineers to spend their time on the work that actually requires them.