Nothing is better than actual experience. This has been repeated by many who have worked in a wide variety of fields, and is why many college courses usually include or end with internships of some kind. It gives hard working students who have been studying for four to eight years in a particular field real world experience in that career. However, just studying and working in one particular field isn't necessarily a good thing. In fact, just about every career out there relies on general knowledge of how a job works, dealing with a wide variety of people in a wide variety of situations. This cannot be taught in any classroom and relies almost one hundred percent on real-world experience.
Which means a really well-rounded worker who is ready for a career in the real world will have worked a few part-time jobs in their lives. These can range from summer jobs they worked in high school to a few shifts a week they worked in college for supplementary income. These part-time jobs are usually minimum wage gigs in various fields, from working behind a cash register in a hardware store to working in a wine cellar to waiting tables to tending bar. Every single one of them is entirely different, tests a different skill set and teaches you things about how the real world operates.
For example, as a waiter, you will have to learn how to present yourself to a new table of customers each time someone sits down. This is almost like a crash course in small amounts of public speaking, forcing someone to get over their fear of talking to strangers repeatedly. Waiters also must do math quickly and figure out the most efficient way to do a lot of jobs at once. All of these challenges will definitely present themselves again in many different fields they go into in the future.
Working a job at a store where you have to help customers find different items and answer their questions not only teaches someone how to talk to those they don't know, but it also teaches them the art of salesmanship. This will undoubtedly come into play in future careers. While a course could probably be devised within the confines of a classroom to convey the basics of these real world lessons, it wouldn't be nearly as effective as the real thing.
While working in a part-time job or internship in your desired field is obviously important and critical to many future careers, the part-time job in something entirely different is important for a broader more well-rounded development of someone who can work in many different situations in the real world. In fact, there's always a chance one of these random part-time jobs leads to a career and passion in something else entirely, one you never would have discovered otherwise.
Maybe you have always wanted to work with animals and there is a job opening at a zoo or pet store. Maybe you have always been curious about art and there is a spot as a security guard at a museum. If you take a part time job that is in a field that isn't your focus and go into it with an open mind, the possibilities are endless. At the very least, it will make a great story later in life.