It's a weird question. But it's how I know whether or not I can deliver value to that potential employee. At big companies, salary and benefits is usually the main "value" that they provide. So, in essence, they provide the higher comp package necessary to make up for the fact that an employee is not going to learn as much as they would at a smaller company. A small company or an early stage startup is full of opportunities to learn. If your goal is to start your own company someday, I can make sure you're leaning the things that will help you get there. If you want to be a race car driver, I don't have much for you.
Employment is a two-way agreement. It's true that employers have the upper hand because they typically represent 100% of that person's income. The employee, on the other hand, is 1/N of the workforce. Switching costs are also likely higher for the employee.
For many people, I would imagine that a transparent and honest assessment is preferred to a pretend make-believe world where we imagine that an employee will spend their entire career in one place.
If I had an interview and was asked that question, I would consider it an enormous red flag, and would either:
* Give you a vague, non-committal answer,
* Give you the answer I suspect you most want to hear (if possible), or
* Walk out.
How could you expect an honest answer to such a loaded question? Fully-loaded machine guns aren't as loaded as your question above.