Whether or not to use props is such a controversial topic in yoga. Some lineages need many, others eschew them all. I understand both sides and do choose to use them with discretion. I use them to aid my students in finding their alignment in the pose to keep their bodies safe. I do know, though, that the more you add props, the less you are connected to your body; the very thing we want to cultivate with yoga.
The musasana pose we studied last month was Sukhasana (Easy, Cross-Legged Pose). In Sukhasana, the prop we use is a blanket under the hips so we sit higher. First, make sure it is truly Sukhasna you are in. There are three main cross-legged seated positions in yoga. In Sukhasana, the feet are away from the body so the shins are almost parallel. Feel are moving toward hidden underneatht the opposite knee. (In the photos that follow, MuseHeather's legs are a bit too close together, causing a torque in her ankles. These photos are expressly to demonstrate the effect of a lift)
Now that you know you are in Sukhasana, should you use a lift? Here is what I know about the psoas: If your knees are above the hip joint, the psoas cannot release. If the psoas doesn't release, the lower back is gripping. Keep in mind, you might be unaware of the gripping!
Here, MuseHeather is not using a lift. Here knees are well above the hip joint.
Here, with the simple addition of a lift, MuseHeather has raised herself above the knees so her psoas can release.
So, this pose is the perfect example of the benefit of using lifts. With the help of the lift, the student was able to be more comfortably in the pose (even if she wasn't fully consious of the benefit.)