Throwaway rooms have existed long before FP and MBs. They've been booked for EMH, ME, dining, etc. In Sept 2013, they were booked at POP (not FW) just to be able to have access to FP+, as POP was one of the first testing resorts. This was largely onsiters who booked these because they wanted to try FP and MBs. I never heard of anyone putting fake people on these, and lots of people even showed up in person to check in. The current throwaways seem to be largely booked for FP earlier access and MBs, but that doesn't mean that throwaways have anything to do with ordering extra MBs for fake people to make SDFPs. As an AP, I could have had extra MBs on my last three trips and used them all of SDFPs without booking any throwaway room at all.
A "throwaway room" is one that is booked and paid for but not slept in. That is different than booking and cancelling or adding extra invisible people to a reservation or trying to get extra days of parking or EMH above what Disney provides.