The mechanics at both of my area Cummins dealer commended me for mounting a boost/egt gage. I mounted a Westach egt/boost gage.
As was mentioned a gage can help with a mechanic or owner diagnosing a engine issue.
They also pointed out when we have 100-115 degree temps with just normal towing at 55-60 mph speeds or especially running in the 70-80 mph range when not towing in those kinds of outside temps it takes a good 5-8 min for egt temps to get down to a normal shut down temp range.
My egt/boost gage has already saved me money.....and its fun to operate the Cummins with them for best mpg.