Externally powered subs can also save us money.
You want limiters? External amps have them.
You want EQ & DSP? External amps have them and more.
You have the money (and space) and you want the best ultimate sub? Get the flagship Funk 18.2 subwoofer, which has.... oh, guess what? That's right... External amp/EQ/DSP.
I think internal amps are only for convenience and limitation in cost and space.