Do it while you are young and energetic, before you are tied down to one place.
Meet new friends from around the world, who you can share interests with and learn new ideas from.
Gain new perspectives on things you normally wouldn't have.
Instead of just visiting for a few days, you are actually living there for four months.
Learn a new language and maybe pick up a few things you didn't already know.
A semester abroad looks great on a resume!
Learn to be fully independent and see what you can do on your own.
Immerse yourself in a totally different culture… you might really like it.
Experience a different education system than your home country.
Something totally different and an unforgettable experience. You will never do anything like this again in your life.