"I've never really had a problem with my shoulder straps sliding outward requiring sternum strap"
There is more to it than simple load stabilization. Up high you can actually put some pressure on the strap. Not so much it pulls the shoulder straps tight, but enough to pick up a couple pounds of load. This has the effect of distributing the load pressure across your whole upper shoulders, not only on your shoulder straps. Especially when overloaded, like at the beginning of a long trip or just after a resupply, it helps manage the loading.
Over 10 pounds, you need a hip belt, even just a strap. The majority of the load should be on your hips anyway…about 60/40 to 70/30…it keeps the weight off your spine. But, your pack should twist with each step or you end up abusing your buttock and upper thigh muscles, if the pack is stiff. This is a huge drawback to a framed pack for me, the ones I used to use in the early 70s. With a semi-framed, internal framed pack or no frame, packed loosely, it is much easier to walk due to the natural twist your body follows (ie,left foot and right arm, right foot left arm.) This needs to be balanced with the need for support, of course.
The twist, especially if you use hiking staffs, is easily apparent if you think about it…the load shifts from left shoulder strap to right as you walk. Stick your fingers under your strap as you walk sometime, to check. The sternum strap pulled fairly tight over your clavicle (just under your neck area) allows this transfer to happen across the front of your shoulders as well as the back, easing the strain. Of course, you always carry the weight, no way around that. But a sternum strap distributes the load better.