Why do my clients need graphs to believe the software works?

I know data visualisations are good at imparting information. They can do so very quickly. The human visual system is the largest single system in the brain. That makes sense.

Several clients I've worked for require visualisations not just for this reason, but as a way to increase confidence in the software. Sometimes that's the only reason for a few of them to exist at all. What do you think is going on here?