Probably because your journal keeps track of the items. On the right hand side of the journal entry, you should see a "check list" of the items and steps associated with the quest.
Not necessarily. Sometimes it's only given as a hint, if at all. And hints you only see if the quest is the active quest and you are on the main screen. So if you really want to know what a particular quest item is for, you have to go through each quest one by one, reading it word for word, and setting it as active and leaving the journal. You can have up to 25 quests - that's a lot of work to hopefully figure out what an item is for.
Whereas if the quest would show in the popup text for a quest item, it is easy and very clear.