Please do not walk into an interview asking the company who they sell their products to, at least not without being prepared first. There are some issues that this question can help highlight. The dictionary definition of a customer is a person purchasing goods or services from another. In certain companies with a stakeholder management type of setup, this term can also refer to the internal customer receiving the goods. This is then a valid question to find out who within the organization the department is servicing. Another is to evaluate the focus of an organization when it comes to more complex customer groupings.
Identify Internal Customers: Some organizations choose to define their stakeholders within the organization as internal customers. This question can then help identify the customers that the department you will be working in has identified. They could have multiple customers or have different groupings of customers in primary and secondary customers.
Focus on Right Customers: In the case where one is paying and the other is using the services, this can help highlight what the organization is focusing on. The most common example is perhaps Google, which have paying customers, the advertisers, but could also be considered to have a customer relationship with the people who use their free services. Perhaps there are also corporate wide definitions, but your group focuses on one specific subset of the market.
Not Knowing the Answer: It is absolutely essential that you know the answer to these questions before you ask them. At least you should have a general idea of the different types of setups commonly used in the industry. This could otherwise be a very embarrassing moment and ultimately cost you the job.
All Questions to the Hiring Manager at Job Interview