The thing to understand about scopes is that yes, pretty much any old optic will have you shooting better groups than with irons. But when you start comparing scopes side by side, the differences become pretty obvious. Look through a Tasco and then through a Trijicon and you'll immediately see the difference. High dollar scopes have better light transmission, more clarity across the field of view, etc. So there's that. But then there's other considerations, too; ones that aren't obvious when just looking at the far wall of the store. For example, my Leupold VX-R Patrol is a hell of a nice scope, and I'll wager that the glass is every bit as good as a Trijicon costing three times more. So why the devil would anyone pay $1200 for the ACOG? Well, because when you fall the last thirty feet off of a fast rope, crash through a roof, and cushion your fall with your rifle, the Trijicon will still be good to go, while the Leupold will most likely be a useless mess. That's not a concern for me, so I'm fine with the Leupold.
There's other things, too, one of the biggest being repeatability. If you shell out the greenbacks for a quality scope, the click values are absolutely right, every time. If you click up twenty-five clicks and then back down twenty-five, you'll be right where you started. Although this seems like a no-brainer, not all scopes can do it. One of the tests that guys put scopes through for review purposes is called "shooting the box", where you shoot zero, then click up say ten clicks and right ten. Shoot another group. Down twenty and shoot another group. Left twenty and repeat, up twenty, repeat, and finally left ten and down ten, which on a good scope will have you shooting right back at zero with the other four groups being right where they should be. Cheap scopes won't do that very well at all.
All that said, if you don't care much about accuracy, shoot mostly milsurp ammo and have never bothered to look up any kind of ballistic table, you may be perfectly happy with a sub-$100 scope.