You may look at the top card of your library any time.
You may cast Dragon spells from the top of your library.
Korlessa turns the top of your library into extra access, letting you chain Dragon spells off the top while developing a green ramp + blue card-advantage engine. You ramp hard in the early turns, deploy Dragons faster than opponents can answer, and grind out value until your fliers and a big payoff close the game.
Built-in card advantage and selection from constantly seeing/casting off the top
Cheap commander that comes down turn two and starts working immediately
Heavy Dragon tribal support gives access to powerful payoffs and lords
Simic ramp lets you cheat out expensive Dragons ahead of curve
Hard to keep down — even after she dies the deck functions as a normal Dragon midrange shell
Only lets you cast Dragon spells from the top, so non-Dragon cards still clog your draw
Top-deck access is hampered by shuffle effects and fetches
Korlessa herself is a fragile 2/2 with no protection
Simic lacks efficient removal and sweepers compared to other Dragon color pairs
Stax and graveyard/library manipulation hate (Narset, Teferi's Puzzle Box) can shut the engine off
Lets you arrange the top of your library to keep Korlessa casting Dragons turn after turn.
Tighten the curve and raise your Dragon count so Korlessa whiffs less often, and add top-of-library manipulation like Sensei's Divining Top, Soothsaying, and Mystic Speculation to control your hits. Layer in efficient ramp (Three Visits, Nature's Lore, mana rocks) to reach bombs like Dragonlord Atarka and Tiamat faster, and include cost reducers (Goreclaw, Dragonspeaker Shaman, Cloud Key) to chain multiple Dragons per turn. Protect your engine with counterspells and a couple of fast game-enders to convert the value lead into wins.