Flank cinches/girths are primarily for roping. When the weight of the steer hits the horn, you need all the help you can get keeping the rear of the saddle down.
Also, many western saddles (we tend to call them 'saddles' in Arizona, and add a description to anything someone rides that is different from a normal saddle) are girthed/cinched with the cinch full forward. Most Circle Ys and some other brands have two D or C rings, allowing you to choose between full, 7/8th, or 3/4 rigging (see Your Complete Guide to Saddle Rigging
for more info).
If you ride with a full rigged saddle, you may need a rear cinch to keep the saddle firm while riding on trails. The farther back the cinch/girth is, the less reason for one behind you.
Australian girths are basically English ones, with an overgirth replacing the rear billet strap. I'd guess it is about 5/8ths rigging, which means there is no reason for a rear cinch/girth. The horns found on some Australian style saddles aren't there for roping, and the saddle isn't a good design for it even if the horn is strong enough.
I have one Australian style saddle with a horn and rings for a rear cinch. I use the horn to pull the saddle forward on its rack, and put my hand next to it dismounting. I figure the rings in the rear could have a saddle bag attached, maybe. I have another, smaller Australian saddle from the same company (Down Under) that has neither horn nor rear rings. I miss the horn about 5 seconds of each ride. I don't give a rat's rear about not having the rear rings. Both are added to make those of us used to 'saddles' feel comfortable when riding some furin-style thingy...