There are two reasons:

1. In general, when where is used as a relative adverb to guide an attributive clause, it is usually unnecessary to add a preposition before it because it is equivalent to "preposition +which" in meaning. For example:

This is their hotel. This is the hotel where they were staying.

There is a safe in her closet with her jewels in it. She has a safe in the closet to store her jewels.

But sometimes we can see the usage of from where, because from is a special preposition, because it can sometimes be used as an object with a prepositional phrase.

Come out from under the table. Come out from under the table.

Therefore, although the relative adverb where is equivalent to "preposition +which" in meaning, sometimes the preposition from can be used before where due to semantic needs. For example:

We climbed onto the roof, from where we could see the whole process clearly. We climbed onto the roof, from which we could watch the parade well.

2. When the antecedent is preposition+locative noun instead of a single locative noun, the leading word of the attributive clause can only be from where.

Add:

There is a preposition from in the attributive clause and the antecedent is the object of from in the clause. The leading word can be where or which (place nouns are not allowed).

The antecedent acts as the adverbial component of the main clause, and the attributive clause uses where as the leading word.

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