They could be simple stick figures on a green/white/black board. They can be fancy PowerPoint slides. They can be material from erudite sources scanned and shown. They can be hand props.
The lack of them creates a barrier between the presenter and members of the audience who expect visual supports. This weekend I attended a lecture by an expert from the Silent Generation. Previous lectures on the general topic given by members of Generations X and Y had had graphics. This one did not.
Although I'm a Baby Boomer, the no-visuals annoyed me. My mind wandered.
Takeaway: If you aren't equipped to provide your own visuals, hire a vendor who can.