Since timing changes have a direct effect on exhaust gas temperature, an egt gauge is a good reference tool to use to dial the timing in. Especially if you are familiar with tuning similar setups, egt can easily tell you if you're running too much or too little advance. From there you have a better and safer starting point from which to tune for max power. On street cars where running conditions change often, an egt gauge is also a good reference to make sure your mixture is staying consistent.

Also its good to know when you're about to burn something up running through 5th gear on the highway...haha