All websites has a traffic that doesn't fit to any ad networks, usually it's bot traffic or any kind of low quality traffic like a no cookies, proxy, vpn, etc.
Fallback url is used for such kind of traffic.
For Fallback campaign we recommend to use your own creatives. Use it as feeder for your websites.
You may add more than one creative.
If your fallback url gets more then 10-15% of your total traffic it means your traffic quality is low or system works incorrectly. Usually it means a lack of linked ad networks. The minimum amount is 3, recommended is 5+. Add more ad networks and check targeting settings of your campaigns.