The Posterior Oblique Sling (POS) comprises of your opposite Latissimus Dorsi to Glute Max as well as the Biceps Femoris hamstring to the same side Glute (not seen in this pic).
This is a functional fascial line utilised in any movement requiring rotation or rotational stability. If you imagine yourself walking or running and take note of what your arms and legs are doing you will find: as your left leg steps through and is behind you, your right arm will be swung back and visa versa as you continue in a forward motion. This is where your POS comes into play on a regular basis. Helping to stabilise your Lumbopelvic region against rotational forces when needed. If you are someone who walks with a "woddle" (over-rotating their pelvis) then you may have dysfunction in this area. As always, better to get assessed as an individual as there are no blanket rules.
If you play a sport that demands a lot of running or rotational movements (particularly golf; lacrosse; water polo; cricket & any sport that requires you to run) then it is even more imperative that you have a functional POS.
In order for something as simple as walking to be a functional movement, one of the things you need is a functional connection between your opposite lat to glut. If there is a break in the chain somewhere you will quite often find that your Sacroiliac Joint (SIJ) will try and compensate by compressing itself to create stability in the system, or your Thoracolumbar Fascia (TLF) will create tension. This is often why you might find the area where your lower back meets your pelvis constantly tightens up, no matter how much you try and release it.
If you are someone who repeatedly gets a locked up SIJ and gets it regularly manipulated or released, you might want to try and find out why it is happening. There will be dysfunction somewhere that is causing the joint to compress itself.
Your body will always find stability when it needs it and joints are no exception. If you don't feed your system with stability after creating more range of movement (stretching/foam rolling/massage/manipulation) then you are feeding a viscious cycle. Your joints/muscles/fascia/ligaments can compress in a desperate call for stability. You need to feed the motor control system the information necessary to correct dysfunction after releasing a dysfunctional pattern or it won't know what to do with the extra range of movement & extensibility.
If it is left open, nothing will change.