I just understood that we have something similar in Russian with "что за". Direct analogue of "was für" but with the word "behind/before (less often by/after/to)" (contextual) instead of "for" in meaning of "a sort of". I think that any other language can also have same "just because" constructs that have some logic only in the target language's framework but look ridiculous for others...
No. When you say "Welche", you're referring to some list of services. Perhaps you saw a list in the newspaper of the company offering different services in different branches, and you want to know which services the current branch offers.
Was für Bücher liest du? [Allgemein]
Ich lese Romane.
Welcher Roman findest du am Besten? [Spezifisch]
Die Leiden des jungen Werthers
Was für Urlaube bieten Sie an? [Allgemein, Ohne Wissen]
Jetzt ist das Wetter schön in England, Frankreich, und Dänemark.
Aber, welche Urlaube bieten sie an? [Spezifisch]
Wir haben günstige Angebote zu allen Urlauben.
Vielleicht wäre Dänemark nett!
Verbs are conjugated the same way for third-personal plural and second-person formal. Second-personal formal is always written with a capital letter (Sie) while third-person plural (sie) is capitalized only if it appears at the beginning of a sentence. In this case, because Sie (with a capital) appears in the middle of a sentence, one can tell that is "you" and not "they." If the pronoun were at the beginning of the sentence, the meaning would be ambiguous.
I'm going to explain it to see from their perspective, to make it sense a bit more:
A - I need a game B - What for? A - For racing purposes B - So, these are the games you can race..
As you can see, B is asking A to categorise his demand. And the idea develops in time to become the structure in the question.