Regarding the "invitations" thing it just makes much more readily understood by the consular officials.

Getting a business visa to enter Australia, although there is no requirement for one I got the company we were visiting in Brisbane to fax an invitation over anyway. I gave them the words. On taking the application in, in person, the consular official leafed through the paperwork, came across it, and said "ah, that makes things easy".

So it seems that a bit of something on a proper letterhead they recognise from their home country helps them.