It gets rid of a lot of crap in the engine that can be caused by many miles of garbage dino oil.
I used the engine/trans flush on my aunt's Forrester when I switched it over to Amsoil, after 65K of dino oil. She went home and called me the next day wanting to know what I did to her car. Thinking something was wrong, she quickly added the thing took off like 10 pissed off gorillas when she got on the interstate. She said it had never ran that well since she had owned it.
Now I don't know what it could've been, but I personally noticed my Taurus SHO ran a heck of a lot cooler after I did the same flush on it and it had already been on Amsoil for 2 seasons before the flush. I always recommend doing the flush when switching to your first change of Amsoil.
Yeah, I wouldn't like the idea of letting the dealer do it myself. However if you do try and do it yourself, it can be a bit messy when you do the flush. It comes out like hot water, not oil and it gushes out!
BTW, I double checked my mileage today and I have over 13K on this oil change, not the almost 12K I mentioned above. The EAO filter is recommended for up to 15K, but the oil is covered to 25K. Most I have ever put on one change was over 17K on my wife's Outback sedan, (170K and still purrs like a kitten). As I said, I"m going to change it again in my Legacy in the next couple of weeks, just for piece of mind.
Besides I just ordered a Fumoto drain valve and gotta get that installed.