The Apple Watch Series 5 combines all the features you could ever want from a wearable band. Every year, Apple has been adding necessary features to make the Apple Watch better. The Series 2 came with the needed power for native apps. After that, Series 3 brought cellular connectivity, and Series 4 had display pushed to full capacity. Series 5 allows for an always-on display. The watch is now a watch.
If you own the Series 4, an always-on display may not be reason enough to upgrade. However, the Series 5 offers much more. Apple is not the first manufacturer to do this, but they might as well be the best. Monochrome veneer, secondary display layer - all these things are not there. You only get an existing watch face; always on. How does it work? With previous versions, the Apple Watch would go into sleep mode when you were not interacting with it. The same thing happens with the Apple Series 5.