TLDR/TLDW:

• Pouring perfectly level concret is not easy, adjusting a pole on 4 threaded bars is easy
• Realigning a slightly shifted pole because of a shifted base is easy with threaded bars
• Airgap avoids rust and moisture buildup

Yup, I've installed countless poles with these, and prefer them for all ease of leveling and keeping water from building up in the base. Trying to set a flange base pole is a royal PITA, usually involves a fair bit of grinding. A good manufacturer also provides a lightweight skirt base to hide the gap and hardware.

We call those bars J bolts, they're basically a threaded rod with the bottom kicked 90°. When you set up the form for the concrete, you have a prefab template from the manufacturer that you secure the bolts to, and it has a way to pour concrete through or around it. When you pull the forms, you set and level your base nuts and washers, and the pole slides right into place, then adjust as needed, and secure with the top nuts and washers. Now if you make your own template and screw it up, or the bolts become misaligned, well, don't do that. Always keep leveling the bolts in intervals as the concrete cures.