当"which"引导的定语从句用于修饰整个句子时，需要用逗号将其与主句隔开。这是因为非限制性定语从句提供了对主句的额外信息，它们并不是主句的必需部分，而且在语法上需要与主句分开。逗号的使用帮助读者理解句子的结构，并区分出主句和非限制性定语从句。例如：

1. The sun heats the earth, which makes it possible for plants to grow.

2. The combination of satellites, which transmit information, computers, which store information, and television, which displays information, will change every home into an education and entertainment center.

在这些例子中，"which"引导的定语从句分别对"The sun heats the earth"和"The combination of satellites, computers, and television"进行了补充说明，并且每个从句的前后都使用了逗号与主句分隔。