Full service removalists, which most people are familiar with (and these are the ones that charge the most) will come and pack everything in your house and then relocate it for you. The difference with DIY removalists (or self pack container removals) is that you do the packing part yourself. A lot of people find that they actually prefer doing this, they get to have more control over what is packed where and also know exactly where everything is. There is loads of advice available on the internet on how to pack a shipping container when moving your household contents interstate and generally people find the process quick and easy to do.
One of the best things about using a self pack shipping container is that you don't have the pressure of having to move everything on one day. Usually the moving container is on hire to you for around a month so you can split that time between loading and unloading. It takes so much stress out of moving to know you're not faced with moving your entire household contents in one day.
Another great thing about DIY moving is the price. Expect to pay around 50-60% of what you might pay a full service removalist. That's a huge saving if you're budget conscious and what to minimise moving costs.