It's a common ingredient in polymeric clarifiers. Because it has many negatively charged sites on the polymer chains it can 'grab' positively charged molecules and clump them so they can be filtered out more efficiently and can also temporarily bond metal ions such as calcium and magnesium so it has a 'water softening' effect. (This is why it is also added to Clorox bleach and some laundry detergent formulas since the larger "clumps do not redeposit as readily on clothes.)

Does it work? As well as any other polymeric clarifier. Overdosing can cause particles to form a colloidial suspension that is almost impossible to filter out instead of forming larger, filterable "clumps"