An employee number is basically a unique identification code/number assigned to an employee of the organization. The organization would generally assign these numbers in a particular format and order. 

Smaller organizations with under 40-50 employees would probably not feel the need for this information, but as an organization grows bigger and over time when it has more and more employees join (&leave), this employee number can be a useful piece of information.

In sumHR, Employee ID/Number is a system generated number assigned to each employee in the organization. It can be used for identification for each human resource, during several different HR operations like payroll, roll call, time & attendance, etc., It also allows the HR team to easily organize and find employees in a large database, using the unique number - instead of remembering each individual's name. 

Another important use for the Employee ID/Number is, it helps HR reduce dependency on any other unique number which could be confidential or require authenticity.