This will set the benchmark by which your end product can be judged. The measurability of this can be in terms of time scales, resources, targets and goals.
Some examples could be to automate the workflow of your organisation, to integrate with other systems or meet new scalability requirements.
Every business is unique. Some may be dependent upon other internal systems and, as such, may have specific requirements of how data should be provided. This could be to interface with another application or for it to make an API call to an online account tool like Xero.
You may also want to define internal processes, and who has access to what? For instance, the data entry team can enter data but cannot approve or delete data, whereas System Managers can enter or approve data but can't delete, leaving only Administrators with the rights to delete.
Defining your requirements helps to identify the behaviour, attributes and properties of the future system. Therefore, the main task of any functional requirement document is to ensure that all stakeholders understand what is needed and what is expected.