Different people will use it for different things. That’s one of the advantages of SmartThings: it’s not a system which says “here are the four scenarios you can use this for.” It can be used with many different devices in many different set ups.
Personally, I wouldn’t use SmartThings for any security applications: it just hasn’t been reliable enough for me. But there are certainly a lot of other people who do find it acceptable for some level of security. And I do use it for some noncritical notification situations, like letting me know if the window in the guestroom has been left Open when it’s going to rain. I have an entirely different system that I use for security which is very reliable, but it does have a monthly fee which a lot of people don’t want to pay.
Lighting control is very popular, but then home automation is also popular with people who are blind who may not need lighting control at all.
If you’re just interested in inspiration, browse the projects category in this forum or the stories section of the official blog. You’ll find lots of really creative ideas there. 
Blog:
Community forum:
.
https://community.smartthings.com/c/projects-stories
And here’s my own project report on my most important use cases: