Because your glasses should not be cloudy.
A dishwasher detergent has a big job to do. It has to cut through and remove all types of baked-on, dried-on foods from dishes, flatware, and cookware, yet leave delicate glasses spot and film-free. It also has to work in different water hardnesses and be safe for your dishwasher and everything you put in there. Finding a great one can mean the difference between getting spotless results the first time and having to run it again — or worse, rewashing items by hand.
In our Good Housekeeping Institute Cleaning Lab tests, we've found that dishwashing powders, gels, and tablets or packs all work well. Which form you choose pretty much depends on your personal preference and how much money you want to spend. Powders are usually the most economical, single-dose packs or tablets are the most convenient and, therefore, usually the most expensive, and liquids or gels fall somewhere in between. In our tests, we see the biggest differences between brands, not forms of the same brand, so if you find a brand that cleans well in your machine and water hardness, any form should work for you!
When we test dishwasher detergents in the GH Cleaning Lab, we apply our own greasy spinach mix to plates, bowls, and flatware. The glasses get coated with milk, beaten egg yolk is brushed on bowls and plates, and coffee cup rims are "kissed" with lipstick. We bake mac and cheese and baked beans on to casserole dishes, fry a hamburger in a skillet and make a serving of oatmeal in a saucepan. Everything gets dried or baked-on for two hours before we load up the two identical dishwashers we use for all our detergent testing.
Post-cleaning, we visually check every item for un-removed food bits and greasy residue and the glasses are closely examined for spots and leftover film in a special light box. Every detergent is tested exactly the same way for more reliable results. Here are top dishwasher detergents according to GH Cleaning Lab tests: