I just took a precision rifle class. No one in their right mind would ever use an M&P15 for this class, but what I've learned about scopes and how they're used could be helpful here.
Why spend more than $100 on a scope?
In the past it was about clarity and quality of the glass. Well, technology moves forward and even the low end companies are making really clear, distortion free, glass. Then it was about durability. That too has become more of a standard. For most of us, even the $200 scope will be plenty durable. Now the difference is more about the quality and precision of the build.
The lower dollar scopes are difficult to adjust and not often repeatable. The best way to say it is the adjustments feel squishy. When you try to adjust your zero, the "clicks" are not very positive. Then, if you move one way too far and have to move back, one click is not always one click. Sometimes it's more and sometimes it's less. This will send anyone into a frustration spiral. The technical term for this is called tracking. If your scope won't track, toss it and get another.
If you're only ever going to set the zero once and never adjust the turrets again, this is probably not an issue for you. However, if you're going to hit anything with precision beyond 200 yards, your scope better track.
Why should I have variable power?
I had reached a point where I didn't see the value in variable power. I mean, you only ever use it at the max power anyway, right? Well, I've learned a valuable lesson on this score; field of view.
Take a look at this picture:
The arrow is pointing to the 18"x18" target where Justin is aiming. You can't see it, but trust me, it's there. Of course it's 1,200 yards away (just shy of 3/4 mile). During this exercise we not only had to shoot these targets, we had to find them. Look a little closer in and you'll see the letters L, J, I and M. If you were using a 24x scope, it would be very difficult to even find these targets. Being able to dial the magnification back to 10x or even 3x makes them much easier to find. Once found, dial it up to max and take the precision shot. Target M is only 325 yards out and could easily be hit with an M&P15 with a 4x or 6x scope.
If you were trying to find a target that was closer than 200 yards and all you had was a fixed 6x scope, it could be challenging. Dial that back to 1x and it's much easier to acquire the target.
Another reason for a variable scope is dealing with mirage. However, that isn't as much of an issue until you get out to 400 yards or so.