Wand movement is one of the most important parts of casting a spell. All that energy isn’t going to go very far if it hasn’t been properly shaped and directed. A spell spoken properly but without the correct wand movement usually fails, though there are some exceptions. For this reason most First Year students begin by learning very simple Charms, such as the Levitation Charm, where the consequence for improperly casting the spell may simply be that the user has levitated the wrong thing.

One thing to note is that wand movements are made regardless of whether the witch or wizard is right-handed or left-handed. The Levitation Charm would require the same "swish and flick" (with a swish to the right followed by the flick) whether the caster was left-handed or right-handed. 

You should always memorize and practice the proper wand movements for any spell you attempt to cast. It’s not enough just to know the incantation! The right words might get you an effect, but improper wand movement will result in an effect you didn’t intend.

The simplest backfires involve the magical energy you were building up just bursting away. This can take the form of a shower of sparks, a loud bang, or even a small explosion. None of the Charms you will learn in your first few years have a risk of serious damage should they fail, but that doesn’t mean you should just lazily wave your wand about and mutter the spell. You could still have minor burns or singed hair to contend with.

Another concern is longevity. If you only use your wand to practice your spells and then put it away for a week, you’ll never obtain the level of endurance necessary to cast for more than a handful of minutes. There may be times when you need to cast many spells over a longer period of time, and if your arm or hand start to get tired you may find your spells backfiring at the worst possible moment.

Flexibility is another important quality every witch or wizard should have. It is for these reasons that every young witch or wizard should practice wand movements daily to build better reflexes in the hand and arm they use for casting. There will be a time where dueling will present itself in a witch or wizard’s lifetime and, depending upon the opponent, they may have to fight for their survival. If one of the combatants’ wand movements lack in fluidity, the duelist who has a quicker hand will definitely have a greater chance of winning.

Over time you will find that you may not need to adhere as strictly to the precise wand motions. You will understand how the energy should flow more intuitively and be able to direct it without more than a flourish of your wand. Again, this day is several years down the road, and in the meantime you will need to conduct the proper wand movements. Without them you can only hope that your spell fails.