Yeah I’ve been confused about this as well, but the impression I get is that it allows you to overlap / parallelize I/O a bit more and optimize overall plot time?
It moves plotting over to the 2nd location after stage 1&2. Can be marginally useful to move from fastest SSD to a slower SSD to free up your fastest SSD for a new plot – but your mileage may vary.
It’s worth mentioning that if you do this and your 2nd tmp dir is on the same drive where you want the final plot to be on, then you should set the 2nd tmp dir to the same dir where the final plot should be. This way you avoid copying the final tmp file to final dir after phase 4. Instead it will just get renamed, which is instant.
I find that secondary temp is most useful if you set it to be your final destination. There space is less of a concern.
(above info was recovered from Reddit, where information goes to be… lost forever once it inevitably scrolls off the homepage… 
)
It feels a bit like a micro-optimization to me, but I’d love to learn more from anyone who is an expert on it!