Because quite simply it won't tell you what the ecu is seeing - as the ecu is responsible for fuelling the engine therefore making the decision of when overboost is occuring, can you not see that this is the FIRST part of the diagnostic process to give an answer to Rustyskull's original question in his first post.(Which is all I have tried to help with) Until you see what the ecu thinks is happening you have little idea of what is going on.So in this instance a dash mounted gauge may tell you the actual boost pressure which is nice if you need to know it,but not necessarily what the ecu thinks is going on.I can't make it any more simple than that, a dash mounted boost gauge will tell you boost pressure,(Within terms of its own accuracy) but NOT tell you what the brain that has control of the fuelling is thinking or doing.If it was an old 200TDI with a Bosch VE mechanical pump which Rustyskull was working with, a boost gauge would be a great help. But its not, a boost gauge will be of help further on when he has seen some live data from it and set the boost to its correct limit.