"but it never harm system "

Yes it does!

Here we go again:

TESTING DOES NOT PROVE THAT SOMETHING IS CORRECT.

The hotter your semiconductors, the faster they age. If you run a device just above the maximum junction temperature (which, by the way, your finger can not feel) the device will most likely fail before the product has outrun its otherwise useful life.

By the way, I once had to pull 5V from a higher voltage which was available for other uses, I distributed the heat by using a 7808 followed by a 7805.

Design, do not test and believe, then

have fun,

Erik