Engineers often have no idea - at least at the beginning - of how a business does its business. We can install the computers and write the code, but have no idea why you need to include a Form 10zy2 with every customer order - or even that you do have to.
An SME provides the insight into how a system has to work - what it has to actually do.
It's up to the designers and coders to create the system that accomplishes what the SME has helped specify.