It makes your Dragon spells cheaper to cast, and once you have enough Dragons on the battlefield, it can actually turn into a 5/5 Dragon with flying when it attacks
Effects like Smothering Tithe will trigger in the first case (when the card literally says draw), but not in the latter, even when the text may instruct you to put cards in your hand from the top of your library
You want to minimize your vulnerability to Skullcrack and Command, which means playing your lifegain cards before turn 3
As such, Gavi is to go-to cycling commander if you're looking for the most "pure" build of cycling
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However, if you do choose a target, and that target is illegal at the time Witch's Mark tries to resolve, the spell won't resolve and none of its effects will happen