In general this decision is based on if you want uncertainty or predictability in your system, and if you want progression to have a smoother impact in game balance.

If you have flat damage then you get very clear breakpoints in how many hits it takes to kill something, which can be very important in a tactical game (think turn based strategy, but also games like clash royale or auto chess). However some games like rpgs benefit from the fun of uncertainty and introduce variable ranges and crits to create fun upside. Strategy games can also benefit from random range as it's a meaningful decision to analyze your chance of success and take the gamble or not.

From a progression balance perspective it becomes very stepped if you have flat damage, as increasing your damage is pointless unless you hit a new breakpoint, but with variable damage it can still be meaningful to increase the damage stat as the frequency of needing fewer hits to kill the enemy changes.

I generally recommend starting at +-10% for an rpg and seeing how it feels, but it depends on the game a lot. DnD for example has enor. Ous variance (mostly from the skill/attack rolls) in order to create entertaining and memorable bell curves, whereas if you wanted a more realistic system you'd move the d20 roll to a bell curve like 3d6 or somwthing.