Modern red dots run for years on a single battery, so it is fair to ask why anyone still bolts on iron sights. The answer is that batteries are only one way an optic can fail.
Lenses crack, emitters get blocked by mud or rain, and mounts loosen after hard knocks. Electronics can also quit in deep cold. Each of these turns a fast optic into a useless tube at the worst moment.
Backup iron sights solve this for a few ounces. Folded flat, they stay out of the way. Flipped up, they give you a usable sight picture through or around the dead optic.
The key is co-witness. With an absolute co-witness, the front post sits right in the middle of the red dot window, so the transition is instant. With a lower one-third mount, the irons ride low in the window and keep the view cleaner during normal use.
Practice the switch. Flip the sights up, find the front post, and break a shot. A backup you have never used under a timer will slow you down when it counts.
A durable folding set like these flip up sights is cheap insurance for any optic-equipped rifle.
Disclosure: this page is run by Ozark Armament, which makes the FBUIS-B.
Folding backup irons are often run under an optic as a co-witness backup. If you're choosing an optic to pair them with, see the Ozark Armament Rhino red dot sight.