I think I found my answer at BBC.co
position of prepositions
Note that in questions the preposition is more frequently placed at the end of the clause. It can also be placed before the relative pronoun where it sounds more formal:
In which street does he live?
Which street does he live in?
He lives in the street where all the houses are surrounded by high fences.
He lives in the street in which the houses are surrounded by high fences
For which organisation does he work?
Which organisation does he work for?
He works for a spy network, about which I know nothing.
He works for a spy network (which) I know nothing about.
Note from examples above and below that putting the preposition at the end of the clause is usually also possible in statements:
The people with whom he worked have all been arrested. (Formal)
The people (who) he worked with have all been arrested. (Informal)
This is the bedroom in which he was murdered. (Formal)
This is the bedroom (that) he was murdered in. (Informal)
Note from these examples, that in statements when the preposition is placed at the end of the clause, we can use that instead of who or which or we can omit the relative pronoun completely