Caches, scratch drives for compilation or whatever other disk intensive process you had, etc.
Heavy industrial models were the ancestors of today's all-flash SAN arrays, later common in hybrid ram+flash. RAMsan was one of the major vendors.
CCP used one for Eve Online. They stuck their SQL server on it.
I think I've seen some in person in 2008 used for, IIRC, some form of coordination in Oracle RAC.
without any idea what a SAN array is was this used as a kinda hybrid ram, where you'd keep all your data on a real harddrive and then on system boot push it all to this where it would stay for years until the next system boot?
No. SAN = Storage Area Network. Essentially networked block devices. a RAMsan array would terminate multiple 1-4Gbit links and provide a very fast block device (possibly subdivided into logical devices) that would be used by other computers.
The contents were persistent, through battery storage + UPSes etc.