Do I need a water filter? 

You may believe your tap water is just fine.  Well water, for instance, is simply great tasting in many parts of the world.  Of course in other parts of the world, it can be pretty bad. But if you've tested your well water (on a regular basis, naturally), and it shows clean and relatively pure, then you probably don't need a water filter.

A lot of municipalities have water that simply isn't pleasant to taste or smell.  The water itself may be perfectly safe, but depending where you're getting your "city water", it could have an odor of sulfur, chlorine, or any other number of unplesant odors. Some municipal water even will have a slight tinge to the color, making it appear unpure.  As well, people are becoming more and more concerned with chemicals which are added to our city water systems.  Many (myself included) just don't want to ingest all the chemicals the city puts in our drinking water to "purify" it.

Whatever the reason, an on-tap water filter is relatively inexpensive compared to other methods of filtration, so is worth a try.  So a water filter is worth the low cost if for nothing else, but piece of mind that the water you're drinking has been filtered.

An alternative to a filter is a home water cooler.  See http://watercoolerhome.com for information about buying or renting your own water cooler for home.