In boot.s of ARM_LPC2106_GCC, why is there a code to overwrite the __data_beg__ with __data_beg_src__ data during initialization of the data in data segment.

I dont know the code but here is my guess. data_beg will be the RAM location that will hold initialised data.  The values to which this data will be set will be in flash at location data_beg_src.  The values are therefor copied from data_beg_src into the variables at data_beg.