Why does my production need a designer? For the same reason you would use a lighting designer, scenic or costume designer and sound engineer.
The special effects designer collaborates with the producing, technical and design team on the script and show content, the choreography or working with the camera crew.
Are you doing a spectacular for 80,000 screaming sports fans or a million people on TV?
Are you designing for a one-time event or an attraction to be sustained for a number of years?
With all the creative, technical and liability factors to keep in mind a special effects designer provides the comfort of knowing the job will be accomplished creatively and efficiently.