It's helpful to know a little bit of classical chinese when trying to understand how words like 所 work. It is used to turn a phrase into a noun so it can fit into more complex sentences without creating confusion in the meaning. The English translation can usually be something like 'that (x) which, those (x) which. Syntactically speaking, the 所 in classical chinese usually represents the object of the sentence.

If you think about the word 所有 meaning 'all', literally what you are saying is 'that which there is', or 'those things which there are'. If we render the phrase 商店所有的货 literally, following the chinese grammar, we get 'that which the store has de goods'.

Often the 所 is optional and is included for extra clarity or else for emphasis, but sometimes it is necessary. Consider the sentence 我 公 司 总 部 在 的 国 家  ('the country where our company headquarters is located'). Using 在 here on its own sounds unfinished in Chinese. This is because 在 is a transitive verb (a verb which takes an object) and very rarely can be used as an intransitive verb (a verb with no object).

However if we add the 所 to the 在, it fulfills the need for the transitive verb to be paired with an object and improves both the flow and the clarity of the phrase:

我 公 司 总 部 所 在 的 国 家

In the sentence given in the question, 这不是你要的 and 这不是你所要的 do not really differ in meaning. If you wanted to make a point of highlighting the 所 in translation, you could translate it as 'that which you want is not this' as opposed to 'this is not what you want', but this really could be considered over-translation.

In short, 所 helps to clarify meaning in relative clauses, particularly where there is no object for a transitive verb.

Hope this helps and more or less covers it, i'm looking forward to others' suggestions

高保罗