Ditto for Visio. Lots of reasons. As an industry standard, it has lots of add-ins available, particularly the technical version. I often have to diagram networks, and the fact that I can find templates for the vast majority of equipment is a great help, e.g., I can draw a rack of equipment that looks like the actual rack because of my Cisco, APC, HP, Dell, and assorted telco templates that look just like the actual product.

Lots of less expensive (some even free) alternatives are probably fine for you to do org charts, programmers' data flow diagrams, simple logic flowcharts, etc. But Visio is really useful when you need something more technical.