its to set up your other skill which deals slash magic damage and in my opinion 1 of the best heal/buffs in the game rune knight class skills are slash damage based
in addition with the buff applied there able to do massive damage and even tank bust cause of pen added. as for the ai my louelle use the skill in gvg and pvp alot when her hp is low or if the hitting a target far range if you have other 5 range skill you will have to turn those skills off for the ai to prioritize using drain evocation