what is purpose this function.???
Nobody knows. When the above statement is torn out of its context it may perform anything.
Go see the TRM, you will find more information about this register.
On your installation directory there is pdf files describing the MCU and all it's registers.
On my computer it is located at \C:\Program Files (x86)\Cypress\PSoC Designer\5.4\Documentation\Technical Reference Manuals\"
Inside this directory there is several PDF for different chip. You have to select the one matching your MCU.