It's not just "gain 5 mana" tho. It's literally an extra turn, which means extra attacks, extra ticks on something like thaurissan or other start/end of turn effects, heck, even the 5 mana alone could definitely mean Mage has alot of spaces to do some fancy combos (Freeze Mage?)
I'm just another one of those. You know, those who do things. Things that need to be done by those that do these things in order to do those things that don't do these things themselves. Has the word 'things' lost meaning to you already?