The mipmap folders are for placing your app/launcher icons (which are shown on the homescreen) in only. Any other drawable assets you use should be placed in the relevant drawable folders as before.
According to this Google blogpost:
It’s best practice to place your app icons in mipmap- folders (not the drawable- folders) because they are used at resolutions different from the device’s current density.
When referencing the mipmap- folders ensure you are using the following reference:
android:icon="@mipmap/ic_launcher"
The reason they use a different density is that some launchers actually display the icons larger than they were intended. Because of this, they use the next size up.