Learn Dutch: Onze schoenmaker, die erg oud is, vervangt het leer. - Our cobbler, who is very old, replaces the leather.

Because this is a non-restrictive (extra information) relative clause. It tells us an extra detail about our cobbler, but we don't need it to know which cobbler we are talking about. In Dutch, just like in English, non-restrictive clauses are separated by commas.

What would happen to the meaning if we removed the commas?

Without commas, it becomes a restrictive relative clause: Onze schoenmaker die erg oud is vervangt het leer. This would imply we have more than one cobbler, and we are specifically pointing out the old one to distinguish him from the others.

Why does the verb vervangt come right after the comma?

It follows the standard V2 (verb second) rule. The entire block Onze schoenmaker, die erg oud is, acts as a single, expanded subject. Since this entire block occupies the first position in the main clause, the verb vervangt must immediately follow it in the second position.

Why do we use die here instead of dat?

The relative pronoun depends on the gender of the noun it points back to. Because schoenmaker is a de word, we must use die. If it were a neuter het word, we would use dat.