the rule on page 21 states:

"If an equipment object of the same name or location is cast upon a Mage, if the spell is friendly controlled, the new equipment object is placed on the Mage, and the duplicate item (the original one which has the same name or location) is removed and placed in its owner’s spellbook. In this manner, friendly Mages can replace equipment on each other with newer or better choices.
"Note that an enemy controlled equipment spell cannot replace equipment on a Mage. In such a case the spell is canceled and discarded without effect."

This means that you can replace your own equipment, since your mage is friendly in regards to itself. This is useful for such purposes as replacing armour that has corrode markers, changing to a new weapon that can counter defences or flying, changing from a spell discount ring to an increased damage ring, and so on.

You can also cast equipment on opponent mages. Generally you wouldn't want to, but equipment like the Eye of Bael can be harmful to some users.