Shadowspear gives an equipped creature +1/+1, trample, and lifelink for a modest investment. Those abilities are useful when a deck can build a large creature but needs help pushing damage through blockers and recovering life. Its separate activated ability also lets you remove hexproof and indestructible from opposing permanents for the turn.
Urza’s Saga can search for Shadowspear because its printed mana cost is one generic mana. Saga’s Constructs are useful carriers: a high artifact count can make them large, and trample prevents a small blocker from absorbing the entire attack. Lifelink can then convert damage dealt into a substantial life swing, supporting a deck that spends life on lands or other effects.
The activated ability does not require Shadowspear to be equipped. You can pay one mana to make an opposing indestructible creature vulnerable to a destruction spell or to let targeted removal reach a permanent that had hexproof. Removing those abilities does not destroy or target the permanent by itself, so another spell or combat result must supply the answer.
Protection and shroud remain different abilities and are not removed by Shadowspear. Its equip cost is two mana, so budget for attaching it as well as casting or searching for it. The card is strongest when both halves matter: a creature that can use trample and lifelink effectively, plus interaction that benefits from stripping away an opponent’s otherwise troublesome defensive abilities.
