This is just a dumb adapter to use Arduino Uno compatible shields with Arduino MKR. There also exists an official MKR2UNO Adapter (https://store.arduino.cc/mkr2uno-adapter ) but I didn't like its layout so I made may own.
There are mainly two reasons for this:
Stacking of boards
The MKR2UNO is layouted that the Arduino MKR sits between the adapter PCB and the Arduino Uno shield. So there is only limited space for the Arduino MKR. On my layout both boards sit on opposite side of the adapter pcb. So you can stack any side as high as you want.
Placement of SPI pins
The Arduino has its SPI functions on pin 11, 12 and 13. Some shields use SPI on these pins and don't use the SPI header. On my adapter these pins are connected to the dedicated SPI pins of the Arduino MKR board. So you can use hardware SPI with the Arduino Uno shields. On the MKR2UNO these pins are connected elsewhere.
Here are images of top and bottom side of the PCB.