Installing one could increase the service life of your water heater.
The extra 40 PSI isn't enough to cause the T&P to release water but in a pressure vessel there is a certain number of expansion cycles that it can take before it tells you replace me!
This is a picture of a failed pressure vessel that had to many expansion cycles on it.
Aloha Airlines Flight 243
Keep flexing that water heater tank and say Aloha Baby!