Commuter Piers can be built only in the Old World / Cape Trelawney.
With the commuter pier your citizens can live on an other island then where they work. This effectively transforms your island workforce pool in a global workforce pool for all the islands of the same world area with a Commuter Pier.
This way it is easier to create islands dedicated for living and for working. The advantage is that you can e.g. keep your pollution low on islands dedicated for living. This has a positive effect on the happiness of your citizens which results in more income. It can also help to make your attractiveness better for some islands.
Building a Commuter Pier on an island with no houses unlocks the chance of city incidents. A Commuter Pier built in an empty island (with only a trading post) has already a small chance of Fire and Riots.
Adjusting working conditions on an island with commuter pier won't affect other islands sharing the same workforce (same as without the Commuter Pier) however, the negative effects by boosting production rate will receive an increased chance of riots and incidents on all islands, scaling up more than on a single island.
Bonus workforce, both from Influence Expansion bonus (+50/100/200 workforce for every tier) and Palace's department of Administration passive effect (+50-300 workforce per Town Hall, for every tier) will also benefit from Commuter Pier.