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I get that they would be aerodynamic in-atmosphere, but so would cigar shaped or pyramidal shaped ones. Is is a rule of cool thing? Preferable to cite some Star Wars reference. Legend/Cannon doesn’t matter, I’ll accept what ever gives a better reason.
Edit: The Star Destroyers in question consist of the following list.
Acclamators, Venators, Imperial I & II, Victory, Super, Xyston (the planet killers, I think), and any others shown on film or in Legends books.
According to a Legends sourcebook, the design focuses firepower forward, giving the ship the ability to destroy a planet.
A Star Destroyer is named after the idea of a ship that has the power needed to destroy entire star systems, an ominous naming convention that goes back to the days of the Old RepUblic. Any ship that follows the design basics of those early ships (including a combination of massive firepower and a dagger shape to focus that firepower forward) can be a class of Star Destroyer. - Starships of the Galaxy (Emphasis mine)
Interestingly, the wording implies that the triangular design is required in order to be classified as a star destroyer, even though some star destroyer types aren't all that arrowhead-shaped.
According to the Star Wars: The Official Starships and Vehicles Collection #11, the triangular shape of the Star Destroyer is simply a case of form following function. Early iterations such as the Acclimator-class and Victory-class Star Destroyers had stubby wings that allowed them to fly in-atmosphere, hence their need for streamlining. They were underpowered, which resulted in later models being redesigned with a bigger power core and bigger engines, but stuffed into a shell that was basically identical to the designs that came before it, even though it no longer needed to be streamlined for atmospheric flight.
The Victory I-class Star Destroyers had a unique profile, possessing 'wings' that allowed the craft to manoeuvre in an atmosphere. While this gave the massive starship the option to pursue prey planetside, the underpowered ion engines of the Victory I meant that the pursued were normally long gone by the time the ship got there. This weakness led to the development of the Victory II-class Star Destroyer, but neither ship saw much service before the birth of the Empire.
Basing her work on her father's plans for the Victory II-class, Wessex designed the Imperial I-class Star Destroyer for Kuat Drive Yards. To address the weaknesses of the Victory-class, Lira Wessex replaced the ageing hypermatter annihilation reactors with an SFS l-a2b Solar Ionisation Reactor.
and
Episode II reveals that the triangular shape of the Star Destroyer is drawn from ships from the time of the Old Republic - specifically the Acclimator-class Republic assault ships.
