Basically, the tools aid concentration and, through consecration and use, build up a psychic power of their own.
Tools can belong to a whole coven, a working partnership or an individual.
You do not need to have all the tools to practise Wicca. Wicca is a religion of improvisation and, because of persecution, many of the tools had to look like everyday household objects or be easily and quickly destroyed. Traditionally the Athame has a black handle and a steel blade, but this is really up to individual covens or personal choice. The Athame is a purely ritual tool so forget the tales about human sacrifice, it is never used to cut. It is interchangeable with the Sword and is used in rituals for casting and banishing circles.