Everything we do, we leave behind a mess that we never want to clean. And that is where we rely on someone to do the job for us. Imagine your house without someone cleaning it every day. Yes, you got it. It may soon look like a garbage truck and will not leave any room for humans to survive.
The same is also true for software development. Imagine the world without a cleaner called a garbage collector. You have developed an awesome software, which is running with all its grace on the most advanced server with a lot of RAM and a large number of CPU cores. In all its glory, it is helping a lot of users get the most out of their software investment. At its peak, inside the application server, your code is loading a lot of classes and creating a lot of objects to support the complex operations your algorithm is supposed to perform...