Very important in a turbo diesel and can also be used to tune the engine for optimum performance. You likely won't see a performance turbo set-up without these gauges installed since EGTs are very critical to longevity of any diesel (esp turbo) engines.
ditto ..
P.S. i thought at least the new trucks turbo diesel need to come already with those gauges .. or at least with light to know over boost and more than safety EGTs !