As we know, Gateway is the technology where we can generate web services that can work based on REST architecture and oData standards. There are many advantages with Gateway services, example you can expose data in JSON format.
Now, if you think there is nothing to do with those benefits then you can create UI5 application without Gateway Services. In fact, you can create it without data/functions from SAP system!
When it can consume data even from outside of SAP & there are many library functions that created for client side code... why should we develop this server side? Does that make sense?
Regards,
Naveen Inuganti