Whether you are a potential buyer or seller, conducting a demo will help answer two questions.
Does the system meet the buyer’s system or technical requirements?
Does the system meet the buyer’s business requirements?
Ideally, a list of system requirements should be available in advance of the demo in order to focus on the functionality that matters most to the buyer.
The reason you also want to know the buyer’s business requirements is to understand if the required functionality aligns to their business agenda (drive growth or improve efficiency as examples) AND whether the desired outcome is significant enough to warrant change.
Often the new system provides the buyer the desired functionality but the extent of the business impact or outcome is insufficient to warrant change. In order to be most productive with everyone’s time a list of both system and business requirements should be known in advance of any demo.