Not only is there all the available info in PDF, but also there is the fact that widespread "print to PDF" facilities make PDF a very accessible standard for personally-generated information that depends a lot on format. For example, I have character generators for HERO System 5th ed. , ForeSight , and HindSight that I built as Excel workbooks. The character sheets they produce are fine if printed to paper or PDF, but unintelligible if copied and pasted to a rich text interpreter. I also have an insanely complicated Excel workbook that implements the star system and planet generation sequences from GURPS Space 4th ed. again, export of the system sheets and planet sheets to paper or PDF is fine, nothing else works.
Obviously, I could get by with e-mail or Dropbox. In fact I have done, playing using a Google+ Hangout, an on-line whiteboard, e-mail, and a wiki. Or I could (and have) use Excel or Word to display a document on my screen, use Grab to convert the screen image to a graphics file, use GraphicConverter to change that from .tiff format to .GIF or .png, and then drag that into a Roll20 handout. But that's not ease-of-use.
