What good are FTP numbers gleaned from an indoor trainer.......to someone who hasn't tested, outside?
First, how would one discern accuracy?
Some people ride better indoors than they do outdoors. Some people ride better outdoors than they do indoors. Some people ride about the same.
Further, some people lose power when transitioning from road bike to TT bike. The amount of power loss varies on how much you ride a TT bike. So if you tested on a road bike indoors but riding a TT bike outdoors, that's another thing to consider.
Accuracy would depend on the device you're using to measure. I have an understanding that my indoor FTP is different than my outdoor FTP so I don't assume they are the same.
Second, what good would an FTP # derived at from an indoor trainer be to someone who didn't train/race (outside) with a PM?
Well, it would help you with your indoor training. It might help you get a better understanding of your RPE which can in turn assist with outside training/racing. A lot of people don't really obsess over power in a race (or they shouldnt, anyway), so it's not much of a loss there but would be a loss for the outside training.
If you train (indoors and out) with power, what are some reasons for power discrepancies (between indoors and out)? If there's a discrepancy between your indoor and outdoor FTP, which is higher? Why?
I mentioned some people get differences from what type of bike they're riding. It also depends on how much they ride each bike. A lot of new people to indoor training vastly underestimate what kind of fan you need to keep you cool, that can impact your numbers.
What are you trying to learn or hope to make a decision on? These seem like broad questions trying to figure out if you should be buying a indoor smart trainer vs an on-bike power meter? Or something else?