The process called dry cleaning immerses garments in liquid. What makes it dry is the absence of water, and that single substitution explains almost everything about what it can and cannot remove.
Water is the problem being avoided
Natural fibers absorb water and swell, and wool in particular has a scaled surface that interlocks when wet and agitated, which is what causes felting and shrinkage.
Tailored garments compound the issue, since a jacket contains interlinings, canvas and padding that each respond to water differently and end up misaligned once dried.
A solvent that does not penetrate fibers the way water does leaves the structure of the garment undisturbed, which is the entire reason the process exists.
The solvent dissolves oils, not everything
Dry cleaning solvents are nonpolar, meaning they dissolve fats, oils, waxes and greases readily while having almost no effect on water-soluble material.
Body oils, cosmetics and food grease therefore leave easily, while sugar, coffee, wine and perspiration salts can pass through the machine unaffected.
This is why cleaners apply spotting agents by hand before the load goes in, treating water-soluble stains with a different chemistry entirely so that the main cycle can do its own job.
Machines recover and reuse the solvent
A dry cleaning machine washes, extracts and dries within a single sealed drum, then distills the solvent so it can be filtered and returned to the tank.
Recovery is an economic necessity rather than an environmental gesture, because solvent is expensive and discharging it is tightly regulated in the United States.
Older facilities used a chlorinated solvent that has been progressively restricted, and much of the industry has shifted toward hydrocarbon and other alternative systems in response.
Pressing is a separate operation
Much of what a customer perceives as the result of cleaning is actually finishing, performed afterward on steam presses and forms shaped to garment sections.
Steam relaxes fibers, the press sets the shape, and vacuum draws moisture out so the shape holds, which is why a returned garment looks sharper than anything achieved at home.
That finishing labor is a substantial share of the cost, and it is the reason a press-only service is offered at a fraction of the full cleaning price.
Why the same garment gets different results
Heat used in spotting or pressing can set a sugar-based stain permanently, so a stain treated once unsuccessfully becomes harder to remove on any subsequent visit.
Telling the counter what caused a stain changes which chemistry is applied first, and it is the single most useful thing a customer can supply.
Care labels are written by the manufacturer to be conservative, so a cleaner will generally decline to deviate from them regardless of what the fabric could actually tolerate.