In simple words, suppose you have a DR and your mother lays the table for you and your homies, you decide what your holiday will suit some dishes. To prepare these dishes you need the products for each dish can be different, and they can be sold in different stores. Oh yeah, mom didn't give you a list of the products she gave you a list of dishes, because you're an adult "developed by information systems" and she hopes that you yourself will understand it. So you run around the shops in search of products simultaneously fumbling Google, which is one dish for 100 or more recipe options(versions). And here you have completed your maneuver, come home, and Mama throws you an exception saying that cheese is not suitable for this version of the dish, and about the sausage you are not forgotten.
But it could be just so, you write it in console composer install and in just a few minutes from you doorstep you receive the courier with a huge number of packages are required for all meals, and you let him in the house, she kindly puts all the packages and tells her mother where that is to accelerate
Mom happy, too, you spent only 5 minutes of your time and most importantly all the products comply with the recipes and you can start cooking))