Diagrams help explain things in a way that words can't. And those things (processes, etc...) will effect everyone - not just the designers.
On the flip side, everyone has to explain something at some point in their lives and/or careers. Having access to a powerful yet easy to use diagraming tool will help everyone to explain the thing that needs explaining.
For example, I used draw.io on Confluence to explain a relationship between original orders and change requests within Salesforce.
The draw.io website has a great article that may help further make this case: Improving Business Processes with draw.io