This spell does take a reaction, so it is best to use it whenever powerful elemental attacks come your way. If you can’t reliably take an elemental hit and have failed the saving throw to half or avoid the damage, then you should probably try to absorb what you can. Note that the spell says the trigger is whenever you take damage, so you can still try to roll your saving throws to avoid the damage before casting the spell. This certainly helps whenever you make a saving throw to half the damage and with Absorb. Elements, you get to half the damage again. If you don’t feel confident in your dice rolls or your HP, then you need to be using this spell to even the odds.