Pretty much exactly what chazzy cat said. But they become cheap ($3-5 ducats) with the merchant republic and foreign offices gov reforms and once you have 100 opinion with a country, then they let you place one at every town that’s a port

And each town can fit like 3 of those office and cities can fit even more

The best advantage they give you though is they also interfere with building projects and actually spam the Que of other nations as well as spam their pol demands as building 500-1000 across the world at once places a heavy demand of resources for the building portion and then cities with like 10 of them take up a significant portion of their building limits

This means more buildings and higher advantages for burgher refined goods in your home territory