"Why is this special tool(t94p-77000-L) so important?"
This is why.
2. WARNING: THE SERVO COVER IS UNDER HIGH SPRING PRESSURE.
Note: To aid in the installation of the servo cover, install a nut with an integral washer (using a permanent thread sealer) onto the forcing bolt of the Servo Cover Remover/Replacer T94P-77000-L.
Install Servo Cover Remover/Replacer T94P-77000-L and compress the servo cover assembly.
3. Remove the servo cover retaining ring and release the tension on the servo cover assembly slowly.
4. After releasing all tension on the servo cover assembly, remove the servo tool.
In addition, the nut with integral washer has part number N621943-S2
"It doesn't look like much and how does it mount to the trans when in use?"
The forcing bolt/nut with integral wahser goes on the center of the servo cover and the other two ends will only fit on two other places.
