The operating system(s) [OS] that your company runs (either on premises or in the cloud) is typically driven by the applications your company uses. While smaller companies should strongly consider running a single OS to reduce support costs, medium to large companies often have multiple operating systems running in their environment. This is to accommodate applications that were developed for a server platform that does not match their desktop OS. The expense is: for each Operating System you run, you will need to staff expertise on that OS to maintain normal business service levels.

Operating System Use Considerations
Linux Linux is commonly used for backend (data center) applications such as networking infrastructure (DNS, NTP, SNMP, etc.).
It also supports a graphic user interface (GUIs) which allows it to be a desktop OS for a small organization to reduce the support footprint.
Linux is easier to harden than many other platforms and there are free hardening guides available on the Internet, which makes it a good candidate for edge services.
Unix UNIX (BSD, AIX, Solaris, etc) has historically run on large servers in the data center for applications that need high throughput (high volume transaction processing) or require significant computational resources. Each version of UNIX has different configuration methods and may require a separate support staff.
Windows Windows was developed as a desktop operating system because the GUI makes it simple for the end-user to navigate to the applications they need. Server versions of Windows were initially developed to support the user environment with services such as file storage, user authentication, and network services such as email servers. As Windows has matured it has added features to run more easily in larger environments. The Office suite of software has also become a common platform for the Information Worker. Small to Medium Organizations (SMORGs) can reduce support costs by running just Windows across their environment.