What is the purpose of sub in this block of code and what are the alternatives to using sub?

Question

.386
.MODEL FLAT, stdcall
.STACK 4096

ExitProcess PROTO, dwExitCode:DWORD

.data
arr1 DWORD 25, 89, 49, 80 
arr2 DWORD 30, 100, 50, 150 

.code
_main PROC
    sub eax, eax
    add eax, [arr1 + 0]
    add eax, [arr1 + 4]
    add eax, [arr1 + 8]
    add eax, [arr1 + 12]

    INVOKE ExitProcess, 0
_main ENDP

END

When I took out sub the sum of adding the values in the array was wrong but I don't understand why. To my understanding sub eax, eax clears the register but I could be wrong. I was told to use something like movsx instead of sub but isn't movsx only used when going between different data type sizes?