There's two computer engineering concepts that you must first understand.
The first is RAM (random access memory). RAM is very fast storage compared to the harddrive. Apps are transferred from the harddrive to RAM when you use a app so you can have fast access to it. What happens when programs use more RAM than you have? Well, some pieces of the program must be paged out into the slower harddrive.
The CPU (central processing unit) usually does the bulk of the calculations that make your app run. Some more advanced operating systems offload graphics processing to the GPU (graphics processing unit). These processors must decode an image and place them on your screen as a bitmap. Think of a bitmap as a huge grid where each point represents a pixel of the image. The larger the image is, the more these processors have to decode and the more they have to work to animate these images across your screen.
Why multiple resolution assets are needed for Android (and iOS too)
Smaller images decrease memory usage. This leads to faster usage of your app because no parts of the app must be paged out to the slow harddrive. If your app uses less memory, it also means more apps can occupy memory simultaneously, meaning faster multitasking. Phones with low density screens are usually the old phones with less memory, so that's why giving a low resolution option for those phones makes the user experience more fluid.
Smaller images also reduce processing time to decode and animate the images on the screen. Again, the phones with low density screens are likely old phones with weaker processors. Providing low resolution images for these phones will make the app run faster.