Why does I start with =1? How does this code work?

function calculate(op) {
    
  if(op === "+") {
    
    let result = 0;
    for(let i = 1; i < arguments.length; i++) {
    
      result += arguments[i];
    }
    return result;
  }
  else if(op === "*") {
    
    let result = 1;
    for(let i = 1; i < arguments.length; i++) {
    
      result *= arguments[i];
        }
        return result;
  }
}