Affinity towards an entropy trigger only builds up as long as the weapon is being held and receiving affinity, but the source of that affinity need not be the weapon itself. Affinity generated by allies, objectives, affinity orbs or the casting of warframe powers all count towards the weapon's affinity and thus charge the entropy trigger.
The burst of damage has the potential to deal 1,750 damage to shields, since magnetic damage has a +75% bonus to them, completely removing shields on all corpus until higher levels in one burst.
Inversely, heavy Grineer with alloy armor will be hardly affected by the blast as their armor reduces magnetic damage dealt to them by 50%.
The fact that entropy weapons can charge and trigger while held but without ever needing to be fired make them uniquely suited to loadouts that prioritize using abilities over weapons, as merely holding the weapon provides additional energy generation. This energy is very useful for caster builds and comes without a tradeoff; such builds will be spending their time casting abilities instead of shooting their weapons anyway, and most other weapons provide no benefit if never actually fired.
Equipping one weapon with entropy gives the player the option of switching to that weapon and generating bonus energy during times when the player is expecting to mostly rely on casting abilities, and switching back to their other weapon when that weapon is needed. For example, a Nidus may carry the Synoid Gammacor while outputting damage via their abilities, and switch to a Vectis Prime if a priority target presents itself outside of reach.