Yes, it must benefit the dealer, but most of all I think the salesman benefits from a generous commission package on these products. Personally, I wouldn't bother, especially as you care for your vehicle anyway.
If you're planning on keeping the vehicle for a few years and having it serviced by Lexus, I'd be more inclined to invest funds in the extended warranty to follow on after the initial dealer supplied warranty. I speak from experience on the warranty as I have benefitted from it on two ISs in the past.
Enjoy your GS if you go through with the purchase.