Bruxas e magos usam varinhas para ajudar a canalizar magia.
Eu imagino que magos europeus como Harry, que estão acostumados a sempre ter uma varinha quando fazem mágica, acham difícil fazer isso de outra maneira. No entanto, em algumas outras partes do globo, as varinhas raramente são utilizadas.
The wand is a European invention, and while African witches and wizards have adopted it as a useful tool in the last century, many spells are cast simply by pointing the finger or through hand gestures.
(Pottermore - Uagadou)
Alguns magos são capazes de fazer magia bem sem o uso de uma varinha
The magic wand originated in Europe. Wands channel magic so as to make its effects both more precise and more powerful, although it is generally held to be a mark of the very greatest witches and wizards that they have also been able to produce wandless magic of a very high quality. As the Native American Animagi and potion-makers demonstrated, wandless magic can attain great complexity, but Charms and Transfiguration are very difficult without one.
(Pottermore - History of Magic in North America)