The only justifiable reason I've heard is they pay it because they want to support the efforts to bring FSD to reality.
For a lot of Model S/X owners it's not a lot of money, and they're not all that concerned.
I probably would do it if Tesla gave updates, and blogged about the progress. If it was like a kickstarter kind of thing where you could feel involved with it. Like I pledged on a glowing plant awhile ago on Kickstarter. Nothing much became of it, but it was highly educational as they kept giving updates on where they were and how they were approaching the obstacles.
My impression of the FSD option was one of disbelief. It was one more thing that made me feel like our very reality broke in 2016 because there was no timeline for any of it. There also wasn't a regulatory or technical path to accomplishing it. Nothing felt grounded about the entire FSD option.
it makes more sense to think about the option emotionally, and not logically.
Emotionally Tesla is really the only car company with any plans to come out with Self-Driving cars for private owners. Everyone else is likely going to stick with fleets because of all the headaches involved with self-driving cars, and keeping things updated/maintained/etc.
I say this is an emotional thing because I don't think it's going to work. It's more of a "lets build it, and see what happens" field of dreams kind of thing.
The other emotional factor is to be a part of something. I'm glad I bought my Model S in 2015 so I could experience Supercharging while it was still in its infancy. It was this fun adventure I can tell my robot kids about someday.
Sometimes in life you do stupid things not because you didn't know better, but because you went along with something ridiculous purely because that ridiculous was also part of something amazing.
Life isn't really about getting from Point A to Point B, but about the fun we had along the way. Money tends to be a distraction from the fun we could be having in life.
I don't think any of us here wasn't completely awestruck with the Falcon Heavy launch. It was quiet a feat, but also a bit silly in a way. To launch a Tesla around the orbit of the Sun.
I'm both a logical person, and an emotional person. With FSD the gap between the two was just too far. I say this as someone who spent $10K on an E-Bike. So it really shouldn't have been that hard to get $3K from me.