Items that aren't identified may have hidden attributes. You can tell if an item needs to be identified by using the appraise command.
Unidentified items may be blessed, cursed, have enchantments, or be otherwise special. Therefore, as it is unclear what properties an unidentifieed item has, it's generally not wise to wear it until it has been identified. To identify an item, either use an Identification Table for 20 gold, or use the Identify spell.
There are a few tricks you can use to determine if the hidden properties are useful before identification:
- Unidentified items can be sold to the store without penalty. Therefore, you can sell all of your stuff, look over the pile for items that are priced abnormally high, and just buy those back for your original sale price to determine which items might be worth identification.
- Some unidentified items are special named items, and these generally have different artwork. In these cases, one might simply recognize the artwork to know what the item will be.