The Brita uses Granular Activated Carbon and Weak Acid Cation Resin (WAC) to remove contaminants. The carbon removes chlorine, chloramine, and to some extent organic chemicals such as pesticides, solvents and pharmaceutical leftovers. The WAC will remove cations such as lead, arsenic copper and more.
Since most municipal water sources have EPA acceptable limits of these contaminants, under normal conditions these filters are not necessary.
However, given the frequent modern day failures of more-and-more municipal water systems, a key function of these products may be as a fail safe, final level of protection before drinking the tap water.
Others may find that water without chlorine compounds in them taste better. Likewise, carbon can remove other things that affect taste and odor.
So, depending on the source water and the needs of the consumer, these products may work, and they may not work. A lot of factors come into play.
I can tell you emphatically, that Brita is a very solid and trustworthy company. If you like the way a Brita or other filter makes your water taste, or if you like the security of a final filter after the tap?