In a mid-size tech-centric product or service company, the IT department is responsible for maintaining and managing the company's computer systems, networks, and software services such as email.
To narrow it down:
Up until 20-30 people, these responsibilities can be split among founders, people ops and operations managers. When you reach a certain company size, it becomes natural to gather IT-related responsibilities and assign them to a single person.
To satisfy the majority of your basic needs in software services, you can use the following software:
Email, Calendar, File sharing, Docs&Spreadsheets, Video chat
Project and Task tracker
Knowledge base and Wiki
The average cost for the services above is ~$25-50 per user per month depending on the plan.