1) Depending on when and where you do your internship, it can help you choose the subjects you need/want to study further along the line.
There are those people who are very lucky and know from a young age exactly what it is they want to do when they grow up, and so knowing what work experience to follow and what subjects to study is always an easier process. Alternatively, if you don’t know what you want to do later on in life, an internship could be an ideal way to help you decide what to study at school or at university. If you do an internship at an accountancy firm and love it, you know you’re going to have to study maths related subjects. In this day and age, we are lucky in the way that it’s not always necessary to study specific subjects to pursue a certain degree. Law conversion courses are an excellent example of this. So my suggestion is, even after one or many internship(s) you are still unsure, to study subjects you enjoy doing and to take a wide variety of subjects from different departments: arts, sciences, languages, social sciences etc.