There are multiple very relevant threats which survive Lightning Bolt but die to Heat, such as [card]Yawgmoth, Thran Physician, Omnath, Locus of Creation, Crashing Footfalls, Primeval Titan, most Planeswalkers, Yorion, Sky Nomad, and the list goes on
They enable surprise plays, counter enemy spells, and provide combat tricks that keep opponents guessing
Rhystic Study is by far the game's best card of its kind, effectively taxing each opponent 1 mana each time they cast a spell Spells are constantly cast in Commander, so Rhystic Study's controller will draw lots of extra cards if the opponents aren't paying 1 extra mana
It also gives you mana towards them, in increasing quantity each time