What you have to keep in mind is that the initial state of your water plays a role in the kinds of filter you will need. If the water is really contaminated with lots of bacteria, or if you want to filter out lots of bad tasting chlorine, using activated carbon instead of plain charcoal.
When you make your own water filter, you know what is in your filter and how it works. This prevents any mishaps as you do not know what is really in a commercial water filter. Apart from that, making a homemade charcoal water filter will also save you a lot of money. You will be able to make it with mostly ordinary household supplies and all you will have to buy is the charcoal or activated carbon.
With the state our tap water is in these days, I do not think anyone denies that we need to filter our water. With homemade water filtration using charcoal and sand, you will know you are protecting your family's health.