Actually tubs, on an interest only loan, it does change the repayment amount. On a P&I loan the contracted repayment amount does not change. The same applies if you use a loan with extra repayments and redraw facilities instead of an offset account. There are exceptions, but this is the most common scenario.
The biggest advantage of an offset account (aside from saving you interest), is that it keeps your savings (which you've paid tax on) physically separate from the loan (to which no tax is applied). This is useful for investors as is separates personal money (the offset) from investment money (the loan), which removes any ambiguity about the purpose of that money and what it can be used for.
Being clear on the type of money in separate accounts ensures that your accountant knows what can and cannot be claimed as a deduction, which may save problems with the ATO.