Why does a guitar have a hollow box?

To keep clothes while traveling.

Not at all.

Wait. I will explain.

Whenever we strike an object, it vibrates.

Sometimes, these vibrations influence other objects to vibrate.

The vibrations which take place under the influence of an external periodic force are called forced vibrations.

So, whenever I see delicious food, my stomach starts to vibrate.

Is this a type of forced vibration?

Please concentrate.

A guitar has a hollow box which holds quite a huge volume of air.

Now, when we strike its string, it starts to vibrate.

These vibrations influence the air in the hollow box, producing forced vibrations in them.

Now we know that vibrations in air produce sound.

Hence, as a huge volume of air is vibrating, the sound produced is louder.