Why do people choose a 3-15 or 3-18 power scope for over a 4-20 or 5-25?  Are those high power scopes for target shooters only?

I've not used more than 10x or 12x on an Ar15 and hunting bolt actions.  I am looking for a scope for a .308 AR.

For me, 4 or 5 seems too much for minimum magnification for how I see rifles being used for utility.

The 20X and 25X are for shots over 500-600 yards, which is the realistic PRECISION limit for most AR rifles, so no need for magnification that high. It simply results in too narrow of a FOV and too much optical distortion in sub $1500 scopes. Those high magnification scopes often have 50mm or 56mm lenses which are again usually too much glass for the AR platform (weight wise IMO). The 5.56 is getting iffy as soon as 300m and past 400m wind makes it an unreliable performer. Sure, you can still make solid torso hits but first round head shots on unknown distance targets becomes a total crap shoot in wind. The .308 is so slow and poor BC, especially in the AR platform, that ranging becomes critical past 500m and wind makes it iffy past 600m. There are usually limitations on barrel length (usually 20" or less), bullets at magazine length (cant load heavy long higher VLD type bullets near the lands and not exceed mag length), etc. that just make 20X to 25X scopes better suited to bolt guns in 6.5 or magnum calibers. If you have a 6.5 Creedoor or 260 Rem AR a 20X scope might have a place, but for 25X and higher magnificiation you really should leave that to 300 WM, +22" 6.5 barreled guns, or other longer range calibers with associated longer barrels.

IMO anything over 10X but under 20X will give you all you need  for precision and 44-42mm objective or smaller allows you to have an optic light enough to shoot offhand easily in a SPR/DMR type rifle role. Anything below 3X magnification is a waste IMO unless you hit a true 1X, plus results in no additional increase in FOV under 3X in many scopes, so it really is a worthless ability in those scopes. You can see that as a black ring entering the view that stops the scope from gaining FOV at lower magnifications in scopes with that issue. Any increase in exit pupil over 8mm is worthless IMO as you gain no additional low light performance. That means a 40mm scope gets full brightness at 5X and a 32 mm scope gets full brightness at 4X. If you want ability under 3X, get a 45 degree offset Delta Point Pro. That is far better than a 2X setting. IMO 2X is just a slow CQB optic and an under-magnified precision optic.

Just my opinions and YMMV.

I went with a 3-15 on my .556 ar. Wasn't planning on ever shooting past 600, and there wasn't much upside to adding more weight it cost for anything with more magnification

My $.02

My experience with higher magnification scopes came from Prairie dog hunting... I have a 32X scope that I thought would be great.. but due to heat mirage.. I had to dial it back to get a usable picture, and every time I did, and looked at whee I ended up?? it was right around 16X

If you want more magnification, expect to pay for really good glass...The majority of us don't get to shoot a lot of really long range... due to lack of places to do it that are close to us. so there is that to consider as well.

The higher the magnification the larger objective lens required to use that magnification.

I have a 3.5-10 and 3-15 on my ARs and shoot them to 1k regularly. 15-18x is plenty good to over 1k. My 6-24x56 on my bolt gun usually ends up at 16x when I shoot long range.

Thanks for the feedback y'all.  Confirmed a lot of what I thought but didn't have the details before!  Between those things and the nearest long range 500+ yard gun range is 2 hours away I don't need to be chasing magnification.