Correct, the GDC1 comes with everything you need, but the MIMOLite is just the device.
The sensor is separate from the laser sensor. It functions as an open/closed sensor, and it’s used in the device type to show the open/closed state of the door; so it’s definitely important.
It’s completely separate from the openers that came with the door. The two don’t interfere with each other, and work fine together. Think of the Telguard/ST integration as another opener but on your cell phone and something that can be automated.
Sure is easier. The intent of the Telguard/ST integration (and others like the Liner/GoControl) isn’t really to replace openers and buttons on the wall. Instead, it allows you to include it in home automations based on activities you want to happen. It could be something as simple as controlling the door when you are away from home, or just knowing if the door is open and to close it from the couch with your phone. Depending upon how integrated you get, you could have it open when a certain car pulls up.
There are several reviews/discussions on the Linear/GoControl device. That’s a good one too, and either device will do the job. It uses a wireless tilt sensor on the door vs the hardwired sensor the GDC1/MIMOLite needs.