Software is main in any system. We always use software as Office suite, Photoshop, etc. but we forget about System on which they are actually running. Application software need perticular environment or architecture on which they run. We commonly use System Software or Operating System are DOS, Windows, Mac/X, Unix, Linux, Ubuntu, Endless, Chrome, etc. for PCs.
On other hand we have Android, Ios, Windows, BlackBerry, Symbian, Java, etc. for Mobile phones.
But why we need OS?
The answer to this is basically we need simplicity in life and if we are addicted to device is due to their simple nature commonly known as UI (User Interface). Due to this we ate having custom UI of every company and thus softwares get divided to perticular companies.
Second most important is to manage memory i.e. management of RAM and Cache data. Through which devices become smooth and perform multitasking.
System software manages all connected and previously connected devices and never forgets them. This is helpful in connecting various gadgets, USB drive, etc.
They also maintain relationships between Hardware and other Softwares. If one need perticular file from hard-disk we call OS and Search in it. And OS searches for file and if it found, shown to user.
OS makes life colourful and with ease. Due to GUI support we are able to use all devices with icons instead of coding or typing.