What is the purpose of your studying?

[humph]

Most likely because he didn't get admission to a PhD immediately after honours, and also because terms in the US tend to start in August/September, so an MPhil allows one to continue doing research work here whilst waiting to go overseas.

I and a friend are doing the same thing (though we will be in the MPhil for less than a year).

Yup, that's pretty much it. To get into US universities for a PhD you have to take the GRE, which is only available to take a few times a year in Australia, and by the time I looked into it, the test was all booked out. So I had to wait until the next application season (the year after honours) to take the GRE then apply for US unis.

I was actually enrolled in a PhD at ANU after I graduated honours, but then changed my enrollment to a MPhil once I was accepted into Princeton.