using assembly below near the end of the code why do we write r3 into r0? how is that not altering our data? and why are we doing this

Write a program that uses a subroutine to find how many 1-bits exists in a 32-bit number. AREA EXPORT ENTRY CountOnes, CODE main PROC LDR re, -0xFOFOFOFO BL Count One Bits _main ; number to be checked stop stop ENDP Count One_Bits PROC LDR rl, -32 ; 32-bit loop counter LDR r3, -0; coun Loop ; 1 in MSB? ; if so, increment count CMP re, #0 ADDLT r3, r3, #1 MOV re, re, LSL #1 ; shift out MSB SUBS r1, r1, #1 BNE Loop MOV re, r3 ENDP END