If you are storing data to disk at that speed, you fill even the biggest optane drives in a couple of minutes. So it would be an application where you need to overwrite a huge amount of data over and over again.
It reminds me of the Bugatti Veyron situation, its tires lasting 15 minutes at top speed yet it runs out of fuel in 12.
As for write speed, you don't really need it unless you're doing file/data recovery, but even then you can just make indexes to the disk image.
AFAIK, very few (if none) are using SCM in their forensic expert workstations, so I can't really tell if you'd saturate the storage capacity or the bandwidth. But in digital forensics, NVMe's have been a must for a while, so this would definitely help.
Another use is to have local storage. You want most of the communication to be between your CPU and RAM/SCM and networking only to transmit results. In case of mongodb that would be something like very heavy aggregation that needs to look at a lot of data but transfers relatively small amount of results to the client (think counting number of objects meeting an arbitrary set of specifications). MongoDB actually requires storage for writing intermediate results of some types of operations so it is not purely about read speed.