You don't have as much of a learning curve, most people have used it and understand the concepts of opening, saving, etc.
Most companies have it. You don't have the "I can't open your file" aspect of things forcing you to constantly save your work, THEN save it again in a different format.
In the insert objects section, is pretty much everything you'd want as far as arrows, connectors, etc. It's not as easy to use as, say, Visio at first. And you lose the benefit of standardized formatting.
But in my years out there now, I have never had someone say "That's not the standard symbol for a ...." It's more speed and accuracy that they want.
I have never used Excel, but I have used Powerpoint. It does a decent job and has some built in flow chart symbols, lines will "auto-connect", etc.
A big factor is how many people will use it. I think Visio charges per seat, too. Most places have the Office Suite which is why I assume Powerpoint and Excel are often used. Visio isn't part of the MS Office suite, so $$$