As others posted, there are many but here are some of mine:
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Because I use mixed range brass with unknown reload history and condition of the brass. I keep separate bins for verified once-fired brass (ones I saw going from factory new boxes to the pistol) and use them for max jacketed loads or practice SD/HD loads (using Remington Golden Saber/Speer Gold Dot HP bullets) for comparable POA/POI practice.
Mixed range brass that's been reloaded several times with hot loads, overly bulged (can you say thinned/weakened case wall?), push-through resized and polished will look no different than many other brass you pick up from the range floor covered in carbon fouling. There's no way I can determine whether the max powder charge I use for the next reloading will be the "final straw" that fails the brass and ruptures the case wall.
I shoot a lot of 40S&W loads which are typically loaded to higher pressures and when using mixed range brass, I prefer to load with powders and charges that won't overly expand the case wall. Since slower burning than Unique/Universal powders tend to lose optimal accuracy and clean burn below high-to-near max load data, I use relatively faster burning W231/HP-38 to develop "plinking" loads at mid-to-high range load data that still produce accuracy.
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Because I want to focus more on shooting drill techniques and multiple targets instead of snappier/heavier recoil. For me, accuracy results from deliberate practice which includes trigger time, both live range fire and dry fire. With lighter recoiling practice/plinking loads, I can better focus on draw from holster/grip/front sight/trigger control/targets. Shooting full-power loads with carry subcompact pistols like G27 and PT145 will result in very snappy recoil.
With lighter loads like 40S&W 180 gr 4.0 gr W231/HP-38 and 45ACP 200 gr 4.0 gr Red Dot/Promo loads, even my wife can comfortably run her shooting drills without worrying about the "flinch" and her wrists will be comfortable after a day-long several hundred round shooting session.
I wondered about different POA/POI from full power vs practice/plinking loads and did some range tests and found Federal Hydra-Shok, Remington Golden Saber, Speer Gold Dots shot about an inch higher on target set at 7-10 yards. So I no longer worry about it. I think more trigger time with lighter load is better than no practice with full-power loads.
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Because I want to decrease the wear and tear on my pistols and extend the life of brass. I shoot a lot and many of my pistols have well over 7,000 - 30,000+ rounds through them. But when I compare my pistol parts to other pistol parts that shot heavier loads, I see a lot more wear and breakage on them than mine. I also believe less the brass gets worked, longer it will last, especially when shot from looser chambers of most factory barrels these days. It is for this reason why I prefer to use tighter chamber barrels like my Sig 1911 and KKM/Lone Wolf aftermarket barrels.
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Because I enjoy doing load development to identify accurate loads and to have more shooting options. While I prefer slower burn rate than Unique/Universal for full-power loads like WSF/AutoComp (and soon CFE Pistol), I found I typically obtain optimal accuracy only at high-to-near max load data with them. With Faster burn rate than W231/HP-38, I have identified accurate enough target loads at mid-to-high range and near max load data.
For me, I like having options and being able to load lighter than 9mm recoil 40S&W loads allows me to introduce shooters to 40S&W more comfortably. Both my wife and sister shoot 40S&W and I used lighter loads to introduce them and allowed them to practice. Now, they can handle heavier 40S&W loads well enough that my sister is thinking about shooting matches with her M&P40. Being able to share the same pistols like M&P45 (with small grip insert) and 1911s with lighter yet still accurate target loads (200 gr SWC with 4.0 gr Red Dot/Promo) with my wife not only increase the fun and joy for range trips but I found she is more inclined to favor more gun purchases if she gets to shoot them as well. (hint, hint)
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Because I want to reduce shooting/reloading costs and ride out the component shortages without using up more popular powders. I like shooting .308 but don't like the cost of shooting .308. After much consideration, I decided to add .300 BLK AR upper so I can have the additional option of shooting .308 projectiles from modified .223/5.56 cases. After even more consideration and research, I decided to load subsonic loads with cheaper powder coated 230 gr bullets. When Missouri Bullet Company offered Hi-Tek coated 245 gr .300 BLK bullets, I was even happier -
http://www.missouribullet.com/details.php?prodId=204&category=6&secondary=&keywords=
I realize 5.0 gr W231/HP-38 vs 4.0 gr Red Dot/Promo difference in 1.0 gr powder cost is relatively insignificant for loading 45ACP 200 gr SWC in overall reloading cost but during two component shortages, Promo plinking loads in 9mm/40S&W/45ACP/45Colt has allowed me to extend the stock of more popular powders like W231/HP-38.
