Because positing them allows you explain things about the rotation of the star or the gravitational fluctuations, what have you.

因为这样假定能帮助你解释,诸如星体运行,引力波动,等现象

耶鲁公开课 - 死亡课程节选

Actually, the view of vectors we take nowadays is that vectors are associated with a pair of numbers which, on the rotation of axes, transform like this.

实际上,今天我们讲矢量所用的观点是,矢量是和一组坐标值相对应的,在坐标轴旋转的过程中像这样变换

耶鲁公开课 - 基础物理课程节选