Learn French: Le sablier de la boîte, lequel est cassé, est petit. - The hourglass of the box, which is broken, is small.

In French, the relative pronoun qui usually refers to the noun immediately preceding it. If the sentence used qui, it might initially sound like the box is broken. By using lequel (masculine singular), it is crystal clear that the relative clause modifies the masculine noun le sablier (the hourglass), not the feminine noun la boîte (the box).

Could we have just used qui anyway, since cassé is masculine and wouldn't agree with boîte?

Grammatically, using qui followed by the masculine cassé would technically point back to le sablier. However, using lequel is a deliberate choice that removes all ambiguity the moment the word is read or spoken, before the listener even hears the adjective. This is a common and elegant way to make sentences instantly clear in French.

What if the box was broken instead of the hourglass? How would the sentence change?

You would change lequel to its feminine singular form, laquelle, to match the gender and number of la boîte. You would also need to add an -e to the past participle to make it agree: laquelle est cassée.

Why are there commas around lequel est cassé?

This is a non-restrictive relative clause, meaning it simply adds extra, non-essential information about the hourglass rather than identifying exactly which hourglass we are talking about. Just like in English, French uses commas to set off this bonus information.