It's a new commercially viable rocket engine (not a pork-barrel jobs program) of a size suitable for serious heavy-lift applications, designed and built in America.
Obviously SpaceX is doing similar things, but Merlin, Raptor, and the BE-4 are all part of a pretty exclusive club. I don't know why SpaceX chose to go with a greater number of smaller engines for their planned heavy lifter; I imagine it might be something to do with the difficulty of building bigger engines scaling non-linearly, and the fact that modern analysis and engineering ought to give us some advantages relative to the last time someone tried to make a really big rocket with a lot of little engines (Soviet N1).
That, and it's a lot easier to shut off 90% of your engines and land at 50% thrust than it is to design an engine that will run at 5% thrust.
Oh, and if your engine is small enough, you can also use the same design on your upper stages - otherwise you need to design two engines.
Also, it may be worth pointing out that the current planned thrust for the Raptor is ~1,700kN at sea level, while the BE-4 is ~2,450 kN. They aren't actually that different - it's just BFR is massive.
The main special thing is that it runs on methane fuel, which has less sooting than kerosene engines. That makes it easier to reuse them. SpaceX is headed in the same direction with their Raptor engine, intended to power the BFR, and Europe is working on Prometheus, which might ship in the mid to late 2020s.
Obviously SpaceX is doing similar things, but Merlin, Raptor, and the BE-4 are all part of a pretty exclusive club. I don't know why SpaceX chose to go with a greater number of smaller engines for their planned heavy lifter; I imagine it might be something to do with the difficulty of building bigger engines scaling non-linearly, and the fact that modern analysis and engineering ought to give us some advantages relative to the last time someone tried to make a really big rocket with a lot of little engines (Soviet N1).