ARM Cortex M3 Systick Question
Not all code you happen to run across is correct and perfect. Indeed the bit should reset itself when it is read. It may not be even get set when trying to set it, so it may be useless to try to set it. Perhaps the programmer who wrote that code did not know the bit resets itself, and incorrectly tries to reset the bit by writing 1 to it, which is the correct way to reset many other flags.