Vraska's Contempt is a versatile removal spell, allowing you to rid any annoying creature or planeswalker at instant speed. Four mana is a lot, though. Plus, the card requires you to be really committed to the dark side as it costs two black mana to cast, making it too expensive for Modern and older formats, but very playable in Standard. When Hero's Downfall was printed in Theros (September 2013), it became a staple and a defining element of the Standard format at that time. Vraska's Contempt costs one more mana than Hero's Downfall, however it does come with additional advantages. For a control deck, gaining two life while trading one for one can easily mean the difference between stabilizing or running out of life points.
The other reason why Vraska's Contempt was important for Standard during its berth was because of the significant number of threats in the format, which were hard to deal with otherwise. The gods of Amonkhet are either indestructible, like Hazoret, the Fervent, or they keep coming back from the dead, like The Scarab God. Being able to permanently remove such powerful threats was extremely important for midrange and control decks. Rivals of Ixalan (January 2018) further increased the importance of contempt with the addition of Rekindling Phoenix, a new heavyweight powerhouse in the recurring threat category.
Energy decks were dominating Standard at the start of Ixalan Standard, but the banning of Attune with Aether and Rogue Refiner weakened the four color versions of the archetype. With the archetype lacking two very good arguments to include green cards, it developed into a Grixis variant, which is less reliant on energy, but uses even more control elements. This new deck tries to survive the early turns with the most efficient removal in the format – Fatal Push, Harnessed Lightning, and Abrade – before taking over the board with high quality creatures. The archetype is still being developed, so the creatures of choice still have some room for changes, but Vraska's Contempt is definitely a staple mainboard inclusion for dealing with pesky threats. All other potential choices have one common denominator though. They will likely result in favorable two-for-one trades, like Ravenous Chupacabra, Glorybringer, Torrential Gearhulk, and so on.