I previously used SFML, XNA, Monogame, etc to create 2d games, where if I display a 100px sprite on the screen, it will take up 100px. If I use 128px tiles to create a background, the first tile will be at (0,0) while the second will be at (129,0).

Unity on the other hand, has its own odd unit system, scaling on all transforms, pixel-to-units, othographic size, etc etc. So my question is two-fold, namely:

1. Why does Unity have this system by default for 2d? Is it for mobile dev? Is there a big benefit I'm not seeing?
2. How can I setup my environment, so that if I have a 128x128 sprite in Photoshop, it displays as a 128x128 sprite in Unity when I run my game?

Note that I am targeting desktop exclusively.