Regular chairs are fixed in size as well as shape, but ergonomic chairs match your body’s needs with adjustment. That flexibility matters greatly. It is improved for daily comfort by it.
A standard chair has no hidden surprises in store. Its appearance does accurately reflect all its functionality. The height is fixed. The backrest is stiff. The arm position, depth, or angle is unchangeable. Adjusting of your posture so that it will fit the chair leads to some discomfort, usually if that chair does not fit to your body.
Ergonomic chairs are made for actions opposite to those. They adjust to you. Until your body feels both relaxed and supported, the armrests or backrest and even the seat can shift or angle and then raise. You are sitting in such a way that it matches to your desk setup and unique shape thanks to this customization.
A regular chair seems like a surface for sitting if comparing both. An ergonomic chair actively supports your health.