Why ac or dc input coupling push button switch is given?

Oh, dude, the reason they put an AC or DC input coupling push button switch is so you can easily switch between AC and DC input signals without all the hassle of rewiring stuff. It's like having a fancy little button that says, "Hey, I can handle both AC and DC, no big deal." So, next time you're switching between power sources, just hit that button and keep on rockin'.