Many RPGs let you use consumable items to attack enemies, create defensive barriers, heal allies, and various other effects. However, as I mentioned in my last post, I find that the extent to which I use such items varies quite heavily depending on which RPG I am playing. In some RPGs, I might use items regularly, while in others I might not use them at all. I am not certain that my experiences are universal, but I do see specific reasons for why I use items more often in some games than in others, and some types of items more than other types.
A major factor that determines how often an item gets used is the rarity and cost of the item. If I can only get a few items of a type, and there are no easy ways to get new items of that type, then I will probably never use that item. I will always "save it for later when I will need it more", and that means never using it at all unless there is a particularly difficult moment in the game in which the item is needed (which has only happened once or twice to my recollection). If an item can be used an infinite number of times, or can be acquired in great numbers cheaply, then I will use it freely.
Another fairly obvious factor is the overall usefulness of an item. I am fairly likely to use a powerful item (such as Final Fantasy's classic "Megalixer" item), especially if it is stronger than other items for its cost. An item which is weaker than other items for its cost is a lot less likely to be used. To use an example from the Final Fantasy series, if I can afford large numbers of both Potion and Hi-Potion, then I will use more Hi-Potion items simply because they heal more than normal Potions.
From this point on my argument gets a bit more complex, so first I will list some specific games (mostly taken from those I mentioned lately) in which I tend to use items, and some games in which I do not. In Chrono Trigger and Final Fantasy games, I only used HP restoration items (mostly only outside of battle), MP restoration items, status restoration items, and Megalixers (only in the final battle). In Atelier Iris 2, I used items of all kinds sporadically. In Ar Tonelico, I only use MP restoration items (and only rarely in boss battles). In Odin Sphere I used many items in every boss battle, and often used items in difficult normal battles. Taking all of this into account, there are a few things I can determine.
First, the opportunity cost of an item is extremely important. In the Final Fantasy series, healing items do not heal more than one person, while other magic spells do, so it is often better to use magic spells to heal many people at once in order to save time that could otherwise be used to do other things. On the other hand, in Atelier Iris 2 only one character can heal the whole party at once (and only with mild effectiveness and at high cost), and there are many healing items that can heal the whole party very well, so healing items are more likely to be used in a pinch than character-specific abilities. Players will tend to go with the most efficient strategies, so items will only be used if they are part of those strategies.
Second, items will be used more if there are effects that can only be created with an item. In many RPGs, this is mid-battle/mid-dungeon MP restoration. In these games, MP restoration items are some of the most important. Even if items are otherwise pointless, items with necessary unique effects will still be used by the player.
Third, the cost of an item is always relative to the cost of other options. In the Final Fantasy series, HP restoration items tend to be useful because it saves MP to be used on other things (such as attack spells), which means that using HP restoration items saves you from using rare and more expensive MP restoration items (since those are the only way to restore MP in a tight spot). In Atelier Iris 2, all character-specific abilities are drawn from a meter that is filled as the battle progresses, so using those abilities has no cost on any permanent resource, making even the cheap and common items of that game have a relatively high cost. Since using the most powerful effect at the lowest cost is the most basic of all strategies, players will tend to use the lowest-cost method to getting any effect.
Finally, there is a psychological cost involved in the ease of use of any particular method. Often, item menus are large messes which require far more tedious navigation than other menus. If it is annoying to sort through, then many players will not bother with it, and use options other than items whenever possible.
As a whole, the reason I used items more in Odin Sphere than in any other RPG I have ever played, even Atelier iris 2, is because everything in the game is designed to encourage item use. Items have a lower cost than character-specific abilties (it can take countless Phozons to refill the Phozon Gauge, practically as many as will be absorbed by the weapon in the stage). Also, items can be replenished easily through the Alchemy Mix system, and have powerful, unique effects that are necessary to winning major battles. Finally, the item menu is rather easy to navigate. Meanwhile, Atelier Iris 2 does many things that discourage item use, most important of which is the fact that items are relatively costly (despite the Mana Synthesis system making every useful item easy to acquire) and unnecessarily powerful (so strong that the basic Flame item can wipe out a whole enemy group).
It is important that games match their systems to the way the game will be played. Players should spend more time and effort on things that will be necessary and useful, rather than on things which will not be useful or are unnecessary. Developers should not waste effort making countless items and systems that will not be used by the player. Items should not become Fake Rewards because they will never be needed due to contradictions in game balance.