Some effects or action consequences may refer to those keywords.
For example: ‘Discard 2 cards with the [Will] keyword from your hand.’
When combining items, keywords also trigger the increase of durability value. You must check the added item according to the first item of the pile (with the die on it).
Remember common keywords are not mandatory to combine items together.
Item A is in inventory, with [Will] keyword and durability of 3.
It is combined with item B, which has [Will + Agility] and durability of 2.
=> Durability of item AB is 3+2=5
Later, I craft the item C with [Agility] and durability of 1. It is combined with item AB.
=> Durability of new item ABC is not changed (value=5), because [Agility] is not shared by the top card (item A, which only has [Will]).