In our experience, shows that have a strong show manager (SSM) are more successful and, therefore, more profitable. The SSM does not necessarily fill that role full time since the show may not be large enough. In that case, the SSM assumes other role(s) throughout the year, such as marketing manager or conference director.
But for each show, there should be a designated SSM to:
Worry about the show 24/7/365. The SSM is always thinking of ways to freshen and improve the show in a "holistic" manner (that is, all aspects of the show including content, relationships, logistics, etc.).
"Own" the market served by the show. The SSM must be an integral part of the "community of interest" served by the show. The SSM belongs to all relevant groups, attends key events, reads every print and online publication, talks regularly to all the industry gurus, etc.
Handle key accounts that are critical to the success of the show. This includes not only exhibitors, but also trade magazines, associations, etc. Communicates regularly with them to form true partnerships.
If your show does not have a SSM, you should appoint someone to fill this role:
Most likely, from the marketing side since they have more of an overall view (as opposed to sales or ops).
An outgoing type, since the SSM has to develop/maintain many relationships (both internal and external).
And note that it is preferable but sometimes not practical for everyone on the show team to report exclusively to the SSM. But at least be sure the SSM is involved in all decisions related to the show.