Good question Joel, not stupid at all. For me, my first use case was that my home had been pre-wired for an alarm system, but no alarm panel was ever installed. So, I had a bunch of unused wires in my walls.
I priced the cost of buying ST battery operated Door/Window sensors. I had 6 doors I wanted to monitor. At the time (2014) that would have cost about $300. Also, I wanted a motion detector and temperature/humidity monitoring in my garage. That would have added another $50-$100. Finally, I wanted to be able to control my 2 garage doors from my smartphone. Adding 2 zigbee/z-wave relays would have been another $100.
So, I was looking at $450 to $500 worth of sensors, most of which would have also required batteries which I loathe changing.
Enter the SmartThings ThingShield. As a hobbyist, it was a perfect solution for me. I was able to buy an Arduino UNO ($25), a ThingShield ($35), and about $25 worth of magnetic reed switches, a DHT22 temperature/humidity sensor, a dual relay module, and a PIR motion sensor. Total parts, about $85.
This project also provided me with a reason and motivation to learn about how SmartThings Device Handlers and SmartApps work. I hacked together an initial solution based on other ThingShield examples I found. It mostly worked, but I knew I could make it better. So, about 2 years ago, my son and I created “ST_Anything” as a library for the Arduino/ThingShield. Our goal was to make it very simple to implement ANY SmartThings Capability with an Arduino and ThingShield. Thus the name, ST_ANYthing.
Recently, SmartThings decided to stop manufacturing the ThingShield. I wanted to make sure that users had an easy to use option still, and thus I worked over the past few weeks to create version 2.x of ST_Anything. This version has support for using Ethernet/LAN connected devices, while maintaining the same look and feel of using the old ThingShield. I wanted to make it simple for users to transition from the old ThingShield, to using inexpensive devices like the $10 NodeMCU v1.0 ESP8266 boards. Had that board been available back in 2014, my $85 project would have only been about $35.
So, for ~$35, you can replace about $500 worth of Zigbee/Z-Wave sensors, with no batteries to ever need replacing.
Hope this provides some insight to my motivation. I know others have created their own sprinkler system controllers, beer home-brewing monitoring and control, control of blinds/curtains, parallel tie-in to existing home alarm systems, replacement of existing home alarm systems, garage door monitoring and operation, water flow monitoring and alarming, temperature monitoring/alarming of fridges and freezers, electrical panel energy usage monitoring/measurements, etc…
For many, home automation is a hobby. It provides an opportunity for them to take their electronics and programming skills and put them to use within their own home. Using an Arduino or Raspberry Pi in conjunction with SmartThings provides a perfect outlet for these skills. Also, one can use a custom solution like this to create something that is not commercially available yet. For example, integration between an old HA system and SmartThings, like an X10 to SmartThings bridge.
Hope this helps to explain at least my motivation.