If a player knows the order of actions, it becomes possible to control the flow of battle very precisely. For example, lets assume a situation where one of the heroes in a battle is very low on hit points. The character will probably die if it takes another hit. The natural course of action would be to heal the wounded character before the monsters get a chance to attack that character again. Therefore, the most logical course of action would be to make the fastest character on the team (or at least a character faster than the monsters) use a healing item or skill on the wounded character. This is a very basic kind of strategy in this type of game.
The only reason I am talking about this is because I recently played a turn-based RPG where the action order was not very reliable: Dragon Quest VIII. While the order of action in Dragon Quest VIII was based on the Agility stat, there apparently was a strong random element that controlled the order in which characters and monsters acted. Jessica had by far the highest agility of any of the characters, and Yangus had the lowest. However, while Jessica did tend to move first, there were numerous times where Yangus would act first in a turn, and Jessica would act last. It was impossible to know for certain who would more in what order during a battle past rough estimations.
This had a decidedly negative impact on my ability to control my fighters. For example, if I tried to do a combo where Jessica cast Oomph (an attack power buff) on Yangus just before he executes a powerful attack, I had to cross my fingers and hope that Jessica actually cast the spell before Yangus moved. Healing mid-battle was similarly hair-raising; I often resorted to having two characters try to heal the same person at once, just to get some assurance the heal connected before the enemy's attack did.
In short, my control over combat in Dragon Quest VIII was not precise. I was limited to controlling actions on a turn by turn level. In comparison, there are RPGs where it is possible to influence events on a much more precise level. In Lunar 2: Eternal Blue Complete for the Playstation, action order was much more predictable. My fastest character always went first in the turn, usually before the monsters did. My slowest characters always moved after the enemies. So I could predict how a battle would turn out, and plan accordingly. I could have my fast character use a resurrection item on a dead teammate, and then be certain that my slower healer character could heal the newly revived hero back to full health on the same turn. In this case, my control over the characters is very precise. I could control events on an action by action level.
Now then, someone could argue that the added randomness to action order makes Dragon Quest VIII harder, and presumably more interesting. However, the added randomness is not the good kind of difficulty. The fun of RPG combat comes from the creation of strategies along the lines of what I described earlier. Good RPG battles are first and foremost a test of wits. Adding wildly unpredictable elements into this battle of wits to make it harder is like making a driving game harder by making the steering randomly stop working every once in a while.
Furthermore, this unreliability can have significant effects on character balance. For example, agility is the only stat at which Jessica is the best character. Her physical stats are all inferior to the other three characters, and her magic power is second to Angelo. Yet, her one best stat is not much of an advantage in a system where luck has as much effect on turn order as stats. So, Jessica sometimes feels like the weakest of the four characters in the game. On the other hand, a character who is reliably fast is inherently useful. The one fast character from Lunar 2 I mentioned above was very useful because of her speed alone, despite not having many strong powers for most of the game.