What dp should I use for Android icons

Question

I discovered this LINK that describes the ideal sizes for Android photos, icons, notifications, etc. I then went in search of some reliable industry-standard icons and discovered Google's REPOSITORY. The sizes of each icon are 18, 24, 36, and 48dp. The images for mdpi, hdpi, xhdpi, xxhdpi, and xxxhdpi can then be downloaded individually.

Why do they provide the four different resolutions when I receive an icon for each of the six directories (mdpi, hdpi, etc.) for each resolution? Which dp is the best, too? Should I choose 48 just because it's the crispest number?