The difference with playing tabletop and this game is that Larian has incorporated the height and dark advantage/disadvantage system. Most of the time when I DM on tabletop I don't give bonuses or penalties for those situations, because they're not really in the rules. However here we have to consider it and it does make a degree of sense. So percentage is just an easier way to broadcast this to the player. I agree the element of the unknown is taken out slightly.
There are rules for it? If you cant see your opponent but knows roughly where it is, you can attack; but with disaadvantage. Same applies for the blinded rules. Creatures that are blinded have disadvantage on all attacks that require sight while all attacks against them have advantage. Which incidentily makes colour spray really good on early levels.
Likewise just having an altitude advatage grats advantage on targets lower then you.
