Props in a yoga class are frequently presented as accommodations for people who cannot yet do a posture. Mechanically they do something more specific than that.
A block changes where the floor is
Many standing postures require a hand to reach the ground while the spine holds a particular relationship to the pelvis, and for most bodies those two demands compete.
Raising the floor with a block lets the hand arrive earlier, which allows the pelvis and spine to retain the intended position rather than being pulled out of it.
The posture is not made easier in any general sense. The demand is relocated from the hamstrings and lower back to the shape the practice was actually asking for.
A strap converts a reach into a connection
Where two body parts are meant to be linked but cannot meet, a strap completes the connection so that the intended muscular action can occur.
Without it, the practitioner substitutes movement elsewhere, most often rounding the spine or rotating the pelvis, to close the gap.
The strap therefore prevents a compensation rather than reducing effort, and the muscular work in the strapped version is frequently greater than in the compensated one.
Bolsters and blankets change the nervous system's task
Restorative shapes are held for extended periods, and holding requires either muscular work or external support.
Supporting the body fully with bolsters removes the need for postural muscles to hold position, which is the mechanism by which those shapes produce a settled state rather than a stretch.
Under a knee or ankle, a folded blanket relieves compression in a joint being held near its end range, which matters most when a position is maintained for minutes rather than breaths.
Walls provide feedback rather than support
Practicing near a wall gives an unambiguous reference for alignment, since contact or its absence reports position without requiring the practitioner to see themselves.
In balance postures the wall permits attention to move from not falling to the mechanics of the shape, which is the part that produces adaptation.
Inversions against a wall reduce the consequence of overbalancing, allowing repetition at a frequency that free-standing practice would not permit.
Where propping is not the answer
Props address positional and proportional constraints, but pain that is sharp, radiating or occurring in a joint rather than a muscle is not a propping problem.
Persistent pain during or after practice, numbness, or any symptom that appears reliably in a particular movement warrants assessment by a clinician or physical therapist rather than a different block height.