We have set up actions that send us information when we want to be informed, we have remote commands that can even restart services as needed and do many other things. So why visualize anything?
While for most this question will seem silly because we know quite well what data we are interested in and why we would want to see that data in a more polished form, some goals that can be achieved might not be that obvious.
Of course, it can be easier to assess the problem when looking at graphs, as this allows to easily spot time when problem started, correlate various parameters easily, and spot recurring anomalies. Things like graphs can also be used as a simple representation, to answer questions like "so what does that Zabbix system do?" That does come handy when trying to show results and benefits to non-technical management.
Another useful area is displaying data on a large screen. That usually is a high-level overview of the system state, and is placed in system operators...