关于你问题，我们按以下思路分析，虽然有点哆嗦，但可能对你的理解更有帮助：

▲where用作关系副词引导定语从句时，由于它本身在意义上相当于“介词+which”，所以其前通常无需再用介词。如：

This is the hotel where (= in / at which) they were staying. 这就是他们当时住的旅店。

She has a safe in her closet where (= in which) she keeps her jewelry. 她在壁橱内装有一个保险箱存放她的珠宝。

▲但有时我们也可见到from where的用法，这是因为from是一个比较特殊的介词，它有时可以后接介词短语作宾语（如from under, from behind等）。例句如：

Come out from under the table. 从桌子下边出来。

▲所以，尽管关系副词where在意义上相当于“介词+which”，有时由于语义的需要，也可在where之前使用介词from——from where其实就相当于“from+介词+which”。例句如：

We went up to the roof, from where we had a good view of the procession. 我们爬上屋顶，从那儿可以很好地观看游行。

Raw sewage is being pumped into the sea, from where it pollutes our beaches. 未经处理的污水被排入大海，污染了我们的海滩。

▲顺便再说说下面这类句子中 from where 的用法：

The blood spread rapidly from where he lay.  鲜血从他躺的地方很快流淌开来。

Start reading from where you left off last time.  从上次停下来的地方接着读吧。

The school was only a few blocks from where she lived. 学校离她住的地方只有几个街区的距离。

这类 from where 相当于 from the place where，即这里的 where 属复合连接副词（也有人叫缩合连接副词）。