A lot of textbooks are not necessary. I know you ask "Why put them on the list?", that is currently an unsolved mystery.
Most textbooks you wont need every day, every single lesson. You very well could get away with using the ones in the library if you want to save money.
You can buy current editions, second hand, for reasonable prices. I probably wouldn't buy them from the university bookshop unless you have to or its a last resort (the mistake many of us made in first year is heading straight to the uni book shop and dropping several hundred when it wasn't super necessary).
It depends on the subject if you can get away with old editions (and how old). Some units also need codes that come with the book for access to online resources, this also varies based on the subject.
Basically, to cheaply buy books:
* Buy them second hand off another student (some university portals/FB selling groups/textbooksexchange)
* Try
http://www.booko.com.au if you can't find them second hand or want them new (these might not have codes if you need that, type in the ISBN)
* You can find PDFs floating around the internet, i dont recommend this because it's illegal but it'd be a lie to say its not something many, many students do.