There are indeed many people who just use an Echo, and that may be all you need. With some Echo models you can also use some Zigbee devices, which lets you use Some sensors as well. All good. 
The main reason to use a dedicated hub rather than just an Echo is if you want to add Z wave devices. If you had a bunch of those already from Wink, then just using Alexa wouldn’t be enough.
The other reason is because you will be able to create much more complex rules with smartthings then you can with just Alexa. But maybe you don’t need more complex rules.
How to Get Started Creating Complex Rules in SmartThings
So basically there are plenty of people for whom just an echo-based system is all they need. Different things work for different people.