Doubt on Modular Arithmetic

printf("%lld\n",((2LL*nCr(n+k+1, k+2))%MOD - n + MOD)%MOD);

I came across answer to an editorial solution which required the use of modular arithmetic. My solution could not pass some of the test cases because of the absence of + MOD in the above code. Could someone explain me what normalization does using + MOD bring. Which rule of modular arithmetic is being followed?

It might be because the value in your expression give negative values, and mod of that would give wrong answer, So adding mod makes the answer in the range of [0…mod] and correct as well.