One way of thinking about JCL is to compare it to a menu in a restaurant.
If you are a customer at a restaurant, you and the other customers don't just walk into the kitchen and start cooking your own dinners—that would defeat the very purpose of going to a restaurant. Instead, from a menu describing all the restaurant has to offer, you select items to make up an order, specifying which entrees you want, which salad dressing you prefer, and any other special requests you have. You then ask the waiter to take your order to the kitchen.
In the kitchen, a team of chefs divides up the work and the appropriate ingredients in order to prepare each dish as quickly and efficiently as possible. While the meals are being prepared, you and your friends can ignore what's going on in the kitchen, engaging instead in dinner conversation, catching up on the latest news. When the waiter brings your meal out, you concentrate on your enjoyment of the meal.