A few reasons I guess.
1 And I really like my band, so I may be biased.
2 In the past, no matter which app I've used, constant GPS tracking on my phone tears the battery up.
3 I also use my phone to play music (only local to the phone, not streaming for the same issue - battery life). If my phone is on let's say 40%. I generally ride in the evenings on my way home from work. Tracking GPS would probably kill it after the 1-2 hour ride. Running the music app may take it down to 30% after the same period.
4 I also carry my phone for safety concerns. If someone or myself get hurt in the woods, need to be able to call out.
5 I also do keep my phone in my camelback. If I get a text or call, I never know if it's important and have to stop to check - which happens way to frequent.
6 Now that I have the band, I can see who may be texting or calling while I'm riding and barely have to slow down to dismiss or reply. If it's my wife, parents or boss and an emergency, they know to text and I can check it on my Band.
7 Lastly, sometimes we do "hot laps" if you will - riding a small area fast, with no backpack. It's nice to be able to use the Band without the need of a phone close by.