BPM isn't just about software; it is about transforming your business.
Before you can make any kind of software purchase decision, you need to understand how you 'want' to do business. Changes to your processes or new formal processes will transform your business. Not only will your company be changing how they do business, your employees will be changing how they work. Some changes may look better on paper than they do in real life. Those decisions need to be reflected in your requirements.
Requirements are the foundation for the design of your solution. Your requirements will be a part of your implementation project plan. Your requirements will help define the scope of your implementation project.
Included in your requirements should be your definition of success. Objective measureable goals make it easier to show success. Management will want to see success. You will need to show that success if you wish to expand your solution to other parts of your company.
Attempting an implementation without a comprehensive requirements document will almost certainly lead to delays and cost overruns. How will anyone implement a software 'solution' if they have no idea what the 'solution' is? The costs of creating a comprehensive requirements document are well worth it and will reduce actual implementation costs.
When looking at software, vendors are out to 'wow' their prospects. They want to blow you away with their technology. What you need to know is 'if their software will meet your solution requirements'. If it will, then 'wow' is OK.
One last thought - a key 'moment of truth' will be how your process interacts with your customer. Make sure some thought is given to your customer in your requirements document.
Does your company work from requirements documents?