Main cases I've seen with mainframes involved network-attached ram disks (actually, even earliest S/360 could share a disk device between two mainframes, so...)
A fast scratch pad that can be shared between multiple machines can be ideal at times.
Makes sense in batch environment - you can lock the volume, do your thing, and then freeing it to another task running on a different partition or host.
Still seems like a kludge - The One Right Way to do it would be to add that memory directly to a CPU addressable space rather than across a SCSI (or channel, or whatever) link. Might as well be added to the RAM in the storage server and let it manage the memory optimally (with hints from the host).
There was no locking (at least not necessarily), it was a shared resource that allowed programs on multiple computers to utilize together (also major use case for RAMsan where I worked with them - it was not about not being able to add memory, it was about common fast quorum and cache between multiple maxed out database servers)
A fast scratch pad that can be shared between multiple machines can be ideal at times.