Shinagawa is a business area in Tokyo that offers many advantages, it is central, it is smaller than Tokyo station and therefore easier to locate the Shinkansen platforms, there is less queue at the counter to activate a rail pass for tourists and in general less queue all together
On the other hand, if you wish to activate your Rail Pass, make sure you go from 9.00 to 17.00 only
and the station can be mega busy at peak hours!! watch out!! the station is set as a mega corridor and it can feel daunting to jump in the flow.