If everything was so simple and smooth - why would developers even bother about the density of screens? The axis itself stretches / compresses the image, if it is too small \ large for the screen of a particular phone, but this, by the way, takes some resources - and for lists with a large number of elements this can become a problem of lack of resources, which means lags.
On top of that, some screens simply won't be able to fit the required dimensions, or they will look with artifacts \ squares \ irregularities - in other words, the consequences can be very different and if you are counting on a wide audience - you must do it for each screen density your icons.
As for the second question - as far as I read, the mipmap is created more for structuring - this is exactly the same directory as Drawable. Each item should be in its place, and not hang out in a pile of other items - let them and similar ones. This is CamelCase of your project. Guideline.
UPD to the question of numbers, official documentation reads as follows:
Alternative drawables
Figure 4. Relative sizes for bitmap drawables that support each density.
For all screenshots, it’s not a problem. Likewise, if you include other bitmap bitmaps, for example, for different applications, for example.
Note: You need to provide the density-specific drawables for the bitmap files (.png, .jpg, or .gif) and the Nine-Patch files (.9.png). If you use XML files, you can copy it in the default drawable directory (drawable /).
To create an alternative bitmap drawables for different densities, you should follow the 3: 4: 6: 8: 12: 16 scaling ratio between six generalized densities. For example, if you have a bitmap drawable that's 48x48 pixels for medium density screens, it should be:
36x36 (0.75x) for low density 48x48 (1.0x baseline) for medium density
72x72 (1.5x) for high density 96x96 (2.0x) for extra-high density
144x144 (3.0x) for extra-extra-high-density 192x192 (4.0x) for extra-extra-extra-high density (launcher icon only; see note above)
At the same time I will attach a separate guide on the icons
