Carpet and upholstery cleaners use heat, water, chemicals and powerful machinery on expensive items that customers care about. Shrinkage, colour run, water damage and chemical reactions are genuine risks, and a standard liability policy may not cover damage to the carpet or sofa you are working on.
Why damage to property worked upon matters
Liability policies often exclude damage to "that part of the property being worked upon". For a carpet cleaner, that is the carpet itself. An extension for damage to property worked upon, or treatment risks, is what responds if a rug shrinks or a sofa discolours after cleaning.
Insurers will also ask whether you take items away for cleaning, as property in your care, custody and control needs to be covered while it is off site.