Replacing the original air box on your car for an induction kit will enable the engine to take in more air, allowing it to breathe more than when paired with standard OE components and helping it to work harder as a result. Engines all need oxygen to help them to work to the highest standards, just like humans, and an induction kit from a brand like K&N, Revo, Forge, MST or Mishimoto will let you get the maximum out of your engine.
By taking in more cold air the engine can work harder at a lower temperature, reaching the optimum conditions to generate maximum power. Induction kits often come in the shape of a cone (offering a greater surface area than the standard flat panel) and can come in an air feed like ITG induction kits or in the form of an open air intake - which, while sounding better, won't supply the best supply of cold air like an enclosed alternative.
If the reason for upgrading is to improve the performance of your engine, then a cold air feed option from Pipercross or Green Filters is the ideal choice; but for that improved sound an open cone kit from K&N Filters is what you need.