MUMPS was designed to function as an all-in-one system for hospital data needs; patient details, lab results, admissions, pharmacy stocks etc, but it soon found itself in use in many other data-intensive settings, such as banking, insurance and telecoms. The original machines it was designed on in the late 60s cost $72,000 (in the 1960s!) and had 4kilobytes of memory, expandable to 64k, with storage on paper or magnetic tape. Imagine running an entire hospital, with dozens of terminals on something like like a Commodore 64. Effeciency in utilising memory, processor speed and storage space was everything, and MUMPS was a marvel of its time - resulting in a rapid spread around the world and vendors creating their own versions at breakneck speed.
The first versions were a stand alone operating system, serving multi-user terminals for programming, database administration and the actual programs for the end users; the medical staff at the hospital. As it grew in popularity and developed, MUMPS was ported to different machines by a variety of vendors, and more commonly ran on top of a host operating system like UNIX and VAX/VMS, even down to DOS and CP/M. The variety of MUMPS implementations that have been developed is quite wide, and sadly poorly archived, with several 'lost' versions such as the IBM VM/370 mainframe version.
In the 1980s/90's when I worked writing MUMPS software for the UK NHS and MOD, the versions knocking around in the sites we supported were MSM, DSM, ISM and DTM. While at the time it seemed like MUMPS was widely used in UK healthcare, it was much bigger in the US, where the entire VA health system was based on a MUMPS suite called VistA, which is still in use today, though it's gradually fading. As Vista is open source, it's found use in developing countries, being a large part of the Indian healthcare system. It's hard to say exactly how prevalent MUMPS is today, it's certainly there in places like EPIC healthcare systems and Intersystems Cache, though the role of MUMPS is usually downplayed, being as dirty a word as COBOL to the bright young things. Like COBOL, it's still there, doing it's intended job regardless of fashion.
I know litte about it, but MUMPS also found use in large banking systems, the Bank of England and Barclays included. One of the few still current implementations is GT.M, which is maintained by FIS, a fintech monster of a company. There were a lot of seriously unhappy nerds where I worked, stuck in the unsexiest tech job imaginable (as they saw it, I loved having my hands on what were considered outdated systems, even back then), with not great pay. The lure of London and a high paid MUMPS job in the banks pulled more than a few of our best coders away. Financial companies are cagey about saying what their platforms are (security through obscurity), but allegedly, some very large banks around the world still use MUMPS derived systems.