Coolant is a simpler event, but it is almost always worthwhile. It grants a structure +1 life, and even preventing just 1 damage is useful - particularly for units like Ember Cog, Blastforge Cog, and champions. Coolant's impact may not be game-changing, so if you need magic, don’t hesitate to discard it. However, if you don’t need magic, it’s usually safe to play it on a unit you plan to move that turn. 
There are additional small benefits, such as preventing engagement damage, but generally, it is a reliable, low-risk event.