The SUM() function takes a range, typically. That is, SUM(G2:M2) adds up everything in the range of cells, instead of going G2+H2+...+M2.
There's no difference between SUM(G2+H2) and just (G2+H2) -- the first one adds the two numbers then takes the "sum" of just the answer, the second adds the two numbers.
If you don't put the brackets around the G2+H2 then order of operations kicks in, and multiplication is done before addition. (G2+H2)*D2*100 adds the numbers as anything in brackets is done first, then multiplies by D2 and 100. Without the brackets, it multiplies H2 by D2 by 100, then adds G2 since the addition would be done last.