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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The 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 ?
What is this used for?
may, among other ways, be translated by
À quoi est-ce que ça sert ?
Strangely linguee.fr does not have any relevant example.