Each spec has an execute mechanic but each mechanic favors different execute scenarios
Affliction: Drain Soul is superior for very long drawn out executes, inferior at executing adds and cleaves
Demonology: Soul Fire/Molten Core is functional for both short and long executes but excels at neither
Destruction: Shadowburn is superior at very fast executes, executes of ads, and cleaveing shadowburn on to a second target, but is inferior for long executes.
same is true is for AoE, all specs have different multi target strategies
Affliction: Best Multi-dot, inferior AoE
Demonology: Best burst AoE, decent sustained AoE, decent on multi-dot, no cleaves.
Destruction: good sustained AoE, Weak burst AoE, Best Cleaves.
Rather than having to spend 2.6~ish seconds per ember for a chaos bolt you spend 1 GCD on a shadowburn and get 5% mana back. Not only does shadowburn itself hit harder than chaosbolt but it also allows you to build embers faster by not having to cast chaos bolt. That's the execute as it works for destro.