It was often used to supplement memory available in cheaper ways or otherwise more flexible. For example many hardware solutions allowed connecting more RAM than otherwise possible to be accessed by main bus, or at lower cost than the main memory (for example due to differences in interfaces required, adding battery backup, etc.)
RAMsan line for example started in 2000 with 64GB DRAM-based SSD with up to 15 1Gbit FC interfaces, providing a shared SAN SSD for multiple hosts (very well utilized by some of the beefier cluster SQL databases like Oracle RAC) but the company itself has been providing high speed specialized DRAM-based SSDs since 1978
The way it makes sense is when you can't add that much memory to the system directly, or when directly attached memory would be significantly more expensive. For this you can get away with much slower memory than you would attach to the memory bus directly - all you need is to be faster than the storage bus you are using.
Last time I saw one was with a mainframe, which kind of makes sense if adding cheaper third party memory to the machine would void warranties or breach support contracts. People really depend on company support for those machines.
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)
RAMsan line for example started in 2000 with 64GB DRAM-based SSD with up to 15 1Gbit FC interfaces, providing a shared SAN SSD for multiple hosts (very well utilized by some of the beefier cluster SQL databases like Oracle RAC) but the company itself has been providing high speed specialized DRAM-based SSDs since 1978