Sitting in an upright position and gently lengthening the spine creates space for the breath to become balanced. A sloppy posture usually means sloppy breathing.
We “posture” in many ways, and the same holds true in that a sloppy posture can translate as a sloppy attitude or a sloppy response. An upright posture usually supports a poised mind and a poised attitude. From this place, you have greater freedom in responding to the varied experiences within a meditation: sensations in the body, emotions that arise, and thoughts that appear.
Coming to meditation with the intention to support altitude and poise, as well as receptivity and presence, supports a lovely mental posture. As we get comfortable in the mind and body, then our emotions often report ease. With comfort and ease, you have established proper asana and you are ready.
Ready for what? All that may be revealed in the very next moment. And the next. And the next.