First of all, what is an internship exactly? An internship is a period of time set by companies where students and graduates can come in and gain work experience first-hand, mainly relating to their field of study. They can be anywhere from a week to a year long and are an excellent way to get an intimate look into a specific industry.
Is the industry for you?
If you are studying at the moment and are unsure whether the industry you are contemplating entering after finishing is for you or not, an internship is a perfect way to find out. Take some time to work in the environment you would, should you go ahead with your plans, and get a feel for if this is somewhere you can see yourself in the future. You may come out of it with either a newfound passion for your future plans or a need to re-evaluate them.
Flesh out your CV
There are few examples of work experience that will damage your CV - except a lack of them. Going through an internship will, if nothing else, put some meat on the bones of your CV, which is highly valuable when you are trying to get your first graduate job later on. Who knows, maybe you eventually will get an offer from your place of internship!
Experience working life
If you are currently in university it is entirely possible that you have never had a job. While that is fine, you really should try to experience the workplace before you enter it permanently. Get a head start on what office politics and behaviour look like, what to wear, how to speak with others and ask for things. etc. These are all things that are not taught in schools but that are important to know in your everyday working life.
Gain new contacts
Ah, the crucial part of collaborating and having an easier time should you become unemployed - a network of contacts. These are people that you have somehow come into contact with, maybe by working with them or just by attending the same event, that you can use for your career in different ways. They can be very useful should you be looking around for a job as they might know of a perfect role somewhere that you do not, they could have other people to put you in contact with, and much more. Be friendly and open to help others during your internship and you should be leaving with new and helpful friends.
Have a great time
Remember the most important thing of all - enjoy yourself. An internship can open your eyes to a career or industry you had never considered before, it can help you meet new friends for life, it could open some really great doors to the future, but it should also be fun and the experience of a lifetime. Choose a company that you can see yourself have fun with, and all of the rest will fall into place.