To unload some of the traffic/stress from the OS server. 

The possibility to develop a react front-end with CSR (Client-side rendering) method would be very interesting. So that OutSystems server only needs to provide the page framework & JS, while the data is fetched through API directly. 

For example a CMS can serve all product pictures for a webshop in different sizes. These shouldn't be cached or routed through OS server, as this provides no extra value/performance and might bring extended load on servers. While CMS systems are purpose build for that assigment.