About Life Steal

I get that life steal is good for melee build coz it triggers by melee hit.
However why is it good for spell builds where damage loss is quite big when you hit melee. Especially archer when hitting melee is quite hard

 

Lifesteal on spell builds is pretty situational, and pretty much the only time you'd melee with a spell build is if you've reduced your attack speed down to Super Slow so you aren't cutting into your spellcasting time.