The post-event survey is one of the most important places where an event planner can collect information.
Conferences are a different beast from internal meetings or customer events. Typically these attendees came with professional and self development in mind. Their goals were likely around networking, industry learning, or resource research. Keep these in mind as you evaluate how the event could have been improved. An exhibitor's objectives are likely different from yours and your attendees, but they will have interesting perspective on attendee interaction, and which parts of the event worked for them-or not.
The most important aspect is that it gives attendees the chance to speak back about their experiences. By paying attention to what attendees, sponsors, and exhibitors are saying, planners are able to continue to grow as professionals and create improvements year over year.