I must admit, as boring as the treadmill can be, it definitely does have some pluses over running outdoors. Of course, the most obvious advantages include being shielded from extreme weather, unsafe terrain, or personal safety concerns. Additionally, here are other things I like about the mill:
Not having to carry anything
I can be quite the pack mule when I'm doing long runs outdoors. At any given point, I might be carrying GU, Kleenex, lip balm, phone, headphones, keys, water, and salt capsules, plus wearing sunglasses, running belt, Road ID and Garmin. In the wintertime, add gloves and hat and all the cold weather gear, etc.
Comparatively, on the treadmill, I can just put everything in the console or on the floor next to me. It's very freeing!!!
Facilities in close range
It's always nice to have the bathroom and water coolers/fountains right at hand near the treadmill. Some of my most miserable outdoor runs have been due to needing either a restroom or hydration refill, but not knowing where to find them!
On your mark, get set, GO!
One of my biggest frustrations with my Garmin is how long it takes to load up. When you're pressed for time and rearing to get moving, there is nothing more annoying than watching the progress bar get to 99%, only to drop back down again.
I like how with the treadmill, you just hop on, make a few selections, and hit the Start button. BAM! No muss, no fuss!
Reliable pace/distance measurements
My Garmin's distance measurements have been WAY off in the past.
Plus, my Garmin only shows average current-mile pace, instead of the actual current pace that most treadmills can display. This makes it more difficult to nail tempo workouts or mile splits.
In general, I consider the treadmill's analytics to be much more accurate than my Garmin. I find it a lot easier to measure speedwork on a treadmill than outdoors.
Customizing the course
Chicago is very flat, so it's hard to do any kind of hill work here. This is about as good as it gets:
I consider the treadmill's incline function to be the only way to do any real hill training here. It's convenient to be able to set exactly when and how much of an incline you want to attempt. The possibilities are endless.
Now, how often I actually choose to practice inclines on the treadmill is another story. "Possibilities" is the operative word here. =D