Think of some of the activities that take place in most companies such as Purchasing, Production, Sales, Customer Service, General Admin, Accounting or Management, they need reliable and usable IT to support them.
Some applications are common to most departments (core applications) such as Office packages which allow writing texts, spreadsheets, presentations or databases, email to communicate both internally and externally. Other applications are specialised (line of business) to a particular department, accounting packages for (yes) the accounts department, Customer Relationship management for sales or Enterprise Resource Planning (ERP) allowing companies to plan and control many of their needs in one place.
IT infrastructure is needed to run all these applications. You need a desktop or laptop PC for the users to enter and read data, a physical network for the data to pass along and through, a server and storage to house data and secure the data with backups. A reliable power supply, internet access to stay in touch with the outside world, that access needs to be protected so you need firewalls to stop the outside world from interfering with your data and the list goes on. You have everything in place you need to manage all these resources, a budget to improve, maintain and replace resources, an inventory so you know what assets you have.