In what aspect and why are people worshipping? When I read about Shar on the Forgotten Realms wiki, my overall impression was "nonsense", "badly created". It is maybe typical for such fantasy worlds but such gods are just badly made. There should be something gray or good or useful (Shadowheart tries to explain what Shar offers, but it's hard to grasp) with her.
If she is like the Christian "Satan" (she is not from her description), then her worshippers should be an extremely tiny amount of people, because Satanism mostly exists in the imagination of members of other cults (like the Christian churches), not in reality. The offer from such gods is just too bad for nearly all people to buy.
What do you mean "In what aspect"? She wants to return Toril to its previously cold, gloomy and barren state; furthermore, she's all too happy to see people suffer from mounting despair. As for why Shar is powerful and worshipped extensively? That is because her divine purview is
large: darkness and the night (among other concerns). Not only is it wise to appease a deity that has power over such a fundamental truth of existence, but, like many worshipers of evil gods, they are either deceived from the start or, more likely, they believe they can "game the system" and rise to the top in a way that ultimately satisfies their own ambitions while simultaneously avoiding disfavor.
