Pre-Order to Avoid Stores
If you pass on a pre-order, but still want an iPhone 11 in September, you'll need to head into a retail store when the phones go on shelves next week.
We probably won't see any lines wrap around entire city blocks, but there will almost certainly be lines at Apple Store locations and some carrier stores and retail spots.
If you want to avoid standing out in the cold/heat in a (potentially) annoying line on the morning of September 20th to buy your new iPhone, you'll want to pre-order.
There's also no way of knowing what kind of stock Apple and retail partners will have available the morning of September 20th.
While some stores could have a bunch of stock, some stores might have uneven stock or no stock at all. In other words, there's a chance you'll walk away from the store with your backup plan or nothing at all.
If you don't want to take a chance, buy your phone online ahead of the release date.