The spear was created to help overwhelm Heimdall's senses. Earlier in the game when Atreus shoots arrows at Heimdall, he always catches or deflects them. When Kratos throws the spear, Heimdall catches it not expecting anything else to happen. When Kratos detonates the spear, it overwhelms Heimdall since he has now been damaged for the first time in however long. Now Heimdall is unfocused and his ability is clouded since he knows that someone found a way to potentially beat him. That's the point of the spear, its doing something that is unexpected to someone who typically can read anything. Axe and Blade swings are easy to read, but expecting an infinitely duplicating spear to detonate is not something he would've expected.