An iron and a steamer are often treated as alternatives, but they act on fabric in different ways and produce different results. The distinction comes from what a wrinkle physically is.
A wrinkle is a set of bonds holding a distorted shape
Fibres such as cotton and wool contain chains held in position by weak internal bonds. Water and heat loosen those bonds temporarily.
When fabric dries or cools while folded, the bonds reform in the folded position and the fabric holds that shape. That is a crease.
Removing it requires loosening the bonds again and letting them reform flat, which is why every method involves moisture, heat or both.
An iron adds pressure to the equation
An iron applies heat and weight directly through the fabric against a firm surface, flattening the weave while the bonds are loose.
That pressure is what produces a sharp result: a flat plane on a shirt front, or a deliberate permanent crease down a trouser leg.
The same pressure is a hazard on pile and texture. Velvet, corduroy and heavy knits are crushed by it, and the flattening does not lift out.
A steamer relaxes without compressing
A steamer delivers hot moisture to a hanging garment and lets gravity do the pulling. The fibres relax and the weight of the fabric draws them straight.
Nothing is pressed, so surface texture, pleats, gathers and pile survive intact. The result is softer and less defined than an ironed finish.
It also cannot create a crease. A steamer removes shape rather than adding it, which is a limitation on tailored trousers and an advantage nearly everywhere else.
Fibre decides which tool suits
Cotton and linen hold bonds strongly and tolerate high heat, so they crease deeply and need an iron with moisture to release properly.
Wool recovers on its own given moisture and hanging time, which is why steaming suits it and why many creases fall out of a suit overnight in a humid room.
Synthetics soften rather than relax under heat, so excessive temperature glazes or melts the surface. Their creases are shallow and lift easily with low heat and steam.
Structured garments are steamed for a reason
A tailored jacket contains canvas, padding and shaped seams built to hold a three-dimensional form over the chest and shoulder.
Pressing it flat on a board destroys that shaping, which is why tailors use shaped forms and steam rather than an ordinary iron on a flat surface.
The general principle follows the construction. Flat things that should look sharp want an iron; shaped things that should keep their volume want steam.