I'm sure there is a practical reason.
For example, a 70% used alkaline battery will perform differently than a 70% used NiMH battery...especially in a flash.
Alkaline batteries are great when they are fresh, but the flash gets slower and slower to recycle as they run out of power. NiMH batteries tend to keep the flash recycled faster, until they are almost worn out...maybe 80-90%.
I used to use Alkalines in my flashes...but I would change them out when the flash would slow down. Then I would take those batteries and put them in house hold type devices and they would work for a long time. They just can't keep up with the huge power draw that a flash needs.
Now I use NiMH (rechargeable) and they are pretty good.
Not to mention that it's environmentally friendly to use rechargeables.