If you can do everything you want to with just the Alexa app, then you don’t need smartthings. I have a bunch of Alexa routines that have nothing to do with my SmartThings account.
However, while the echo routines have a lot of functionality, they definitely don’t match smartthings yet. In particular, you are pretty limited in the trigger conditions you can use. You can’t use a switch or a button to trigger routine. And the “if“ side of the routine is much less complex.
So basically, if an echo routine works for you, great. Use it. 
See the following for more details on the differences between the two platforms: