I saw that those were available but I went with the Salsa Mukluk, have put 30 miles on it in 3 days and love it. The tires look ridiculous but actually aren't bad on pavement either. Why do people use those under the seat bags and not a rear rack? Is there something I am missing? I have a set of Ortlieb panniers.
Thanks again for your help.
The Mukluk is an awesome bike, I'm so glad you love it!
Seat bags are a popular choice for a couple of reasons, but mainly because they are more streamlined and keep your weight centered over your bike better than panniers, which put the bulk of your weight over your back tire. Mainly the advantage comes when riding on technical terrain or trails where you're working hard to keep your bike centered and upright.
The drawback to a seat bag is that it holds significantly less volume than a set of panniers (some of the biggest seat bags come in around 15 liters and a set of Ortlieb Backroller Classic Panniers hold almost three times that!). That's why I pair my seat bag with a frame bag, a down tube bag, front fork mounts, and handlebar mounts. I have to be careful about where I put the heaviest items (preferably in the frame bag) to keep the weight evenly distributed over the wheels. My seat bag usually has my tent and maybe a change of clothes (light and packable). My food, water, and stove go in the frame bag, and my sleeping bag and pad go on my handlebars (again, light and packable). If I'm carrying more water or food I'll strap that to my front fork.
All in all it comes down to personal preference. At the end of the day what matters most is that you're out there riding your sweet bike!
Hope this helps, thanks!