Software requirements are necessary:

• To introduce the concepts of user and system requirements

• To describe functional and non-functional requirements

• To explain how software requirements may be organized in a
  requirements document

What is a requirement?

• The requirements for the system are the description of the services
  provided by the system and its operational constraints

• It may range from a high-level abstract statement of a service or
  of a system constraint to a detailed mathematical functional
  specification.

Requirements engineering:

● The process of finding out, analysing documenting and checking these services and constraints is called requirement engineering.

● The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed.

● The requirements themselves are the descriptions of the system services and constraints that are generated during the requirements engineering process.