Like if you pick up an unfamiliar tool in a hardware store and ask the clerk "What is this for?"
What would be a common way of phrasing that?
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Sign up to join this communityThe first expression that comes to me is:
À quoi ça sert ?
Ça sert à quoi ?
You could also say:
À quoi cela sert-il ?
Quel est l'utilité/l'usage de cet objet ?
EDIT
What is this used for?
may, among other ways, be translated by
À quoi est-ce que ça sert ?
https://www.collinsdictionary.com/dictionary/english-french/what-s-this-used-for
Strangely linguee.fr does not have any relevant example.
https://www.linguee.fr/francais-anglais/search?source=auto&query=What+is+this+used+for%3F