Hi and welcome
Sorry I don't know about 3G in Aussie but there are some people on this board who should know so hopefully they will advise.
The keyboard is used for quite a few things. You can search within books (useful when you can't remember who a character is) and you can search across your entire book collection - I use this mainly for finding recipes I want to cook. Very handy.
I also use the keyboard for making notes in a book and for writing online in an online notepad.
Also I write gmail emails with the keyboard.
There are other things you can use the keyboard for including maths calculations but I've listed some of the main ones.
You can use the text-to-speech function if the publisher has allowed it to be enabled on that book. Insometimes find it useful like if I am busy ironing but it isn't as nice as listening to a proper audiobook as it sounds a bit robotic. You can use the kindle speakers or headphones.