I'm not going to get too into this, but in the higher levels on a 'certain' class you will need to prepare your ability rotation based on the circumstances of both the type of damage, and the support function. For instance say there is an AOE ability you need for large groups, it doesn't do as much damage as a single target, but it does a lot more as an AOE in groups (Note damage can be swapped out for enmity, buffs etc.
NDA and all), but this AOE requires MP instead of TP, now lets say you had a combo skill that merges off of a starter weaponskill that also has a more damaging variation that gives MP back based on the amount of damage you do. In order to get MP back for your AOE, you need to do the less damaging single target move to get MP back, either for the AOE move or hell to heal yourself if you are solo. Thus there is a calculated usage of your ability based on the encounter. Say a boss is by himself but then calls adds, you switch from single target rotation to AOE, or you stop fighting the boss and do mass damage single target to the adds to preserve MP.
On top of this all classes have a single target, instant cast range weaponskill/spell (the water spell you mentioned) which have other usages such as sending mobs into traps or even springing traps from a distance, this too is on the global cool down. Basically in the field and fighting single target mobs solo, you won't see too many of these situational abilities used, but they become a dynamic requirement in bosses and dungeons, and are honestly really fun when you get in the flow of things.