This has been a discussion in several games and I even brought it up in the "science" of magic.
Magicked and enchanted items have several uses, from boosting abilities to acting as storage devices for spells.
The "science of magic" explains not only how items are used but also how and under what specific conditions they work.
Example 1: A magic ring -when worn it will conform to the possessor's finger and function as designed. The concept is simple enough, but what if the possessor slips the ring over a glove or gauntlet? What if he is wearing biological warfare gear? Does the ring function due to being worn and in proximity or must it be touching skin? Most rules concerning magic items don't delve so deeply but those interested in the science involved should also be able to explain the whys and hows of magic.
Example 2: Magic brooch... Unless your character is into extreme body piercing, this item is normally worn pinned to clothing and often used to attach cloaks. Under normal use, there is no skin contact, so it is safe to assume it functions by proximity to the wearer. So, as with the ring example, just how close in proximity does the user have to be? If the user is wearing plate armor over a padded gambison, a wool winter tabard and has the brooch affixed to a wool cloak, that proximity may be six inches of metal and material, so... how close does that item have to be to work?
I played in a campaign that required powerful magic items to have contact with the user's skin. That resulted in players having items surgically implanted or designed to be inserted into body orifices...such as the "under my kilt" ale tap.
Some items act as batteries, amplifiers and capacitors that can be used immediately or need to attuned to a character's internal power flow. Some call this ki or chi, others personal Ley Lines, etc.
Some players want to know how their stuff works other than the "handwavium" answer.