Depending on where you live, tap water can be absolutely awful and only marginally suitable for aquarium use. It may be full of chlorine, chloramines, nitrates and phosphates but at least these are generally known about. Rainwater has long been used for aquaria but can be a problem if you live under a busy flight path or have a new roof, as you just don't know what's likely to be in it.
You can use an internal filter packed with activated carbon or a resin pad such as Polyfilter, but for most urban and suburban fish keepers it's safer to use tap water. For the ultimate in flexibility, however, you can use RO water with added buffers to ensure stability and suitability.