You might recall from grade school sometimes you put things in parentheses so that your addition happens first, then your multiplication, otherwise multiplication would happen by default first.

你们可能会想起小学时，将一些加法表达式,放在圆括号里，这样加法可以先计算，然后是乘法，否则，乘法默认先发生。

哈佛公开课 - 计算机科学课程节选