Do you mean the most popular use cases? Smartthings is an extremely versatile system, so different people will use it for different purposes. For example, someone who lives alone might want a “light follows me” type use case where the lights keep coming on and going off as you move from room to room. But as soon as you have two people living in a house, that type of smartapp becomes much harder to coordinate and you’re more likely to have one person sitting there in the dark. 
I myself am quadriparetic with limited hand function, so I am very interested in these cases that give me hands-free operation. The Amazon echo was a very valuable addition to my set up for me for that reason. But if you have a new baby, you may not want to use voice control at all in order to make naptime last as long as possible!
If you want to think about some of the things you can do with automation that you can’t easily do without it, that usually falls into the following big categories:
One) put things on a time schedule. For example, I have an entry light that comes on 20 minutes before sunset. I never realized how much we would like that Until we had it.
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trigger one set of devices from values measured by another set. This could be something as simple as a closet light coming on when the doors opened. But it might also be triggered by motion, light level, a pressure sensor, humidity, a device crossing a geofence, etc
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Group devices in a way you couldn’t do before. For example, I have a “bedtime” routine which turns on one light in the living room, one in the hallway, and one in my bedroom. That way when I’m ready to go to bed, I turn the lights in three rooms on and after I’m in bed I turn them all off again. I do this by voice, but you could do it with one button on a handheld remote or just with your phone.
And there’s the classic “movie night” where you have turning on your home entertainment system also turn down the lights. So sometimes grouping means multiple devices come on together, and sometimes it means some come on and some go off. 
- get notifications when specific events happen. I get a notification when the linen closet has been left open for more than two minutes, or if the guestroom window is open and rain is expected. Different people will find different notifications useful, but most people find something in this category helpful.
Anyway, those are just some General ideas. If you’re looking for an introduction to how to set up these kinds of actions in SmartThings, see the following scheduling intro: