Cerulean blues is an extremely important color. Besides being a core blue (it should be on every palette), it forms complementary tones with most orange-brown colors, such as burnt sienna (another very useful color).
Its usefulness is unfortunate because it’s so blasted expensive–one of the most expensive paints on the market. My oil paint brand reference is keyed on cerulean blue because its cost provides a good metric for comparing manufacturers. Cerulean’s cost is an important factor in a manufacturer’s overall rating.
Because I mix it with a lot of colors, (I use a lot of orange-brown colors), I never buy the high-priced brands but restrict myself to the more affordable ones. Which is why I was extremely disappointed after buying a large tube from Blue Ridge Paints only to discover it was a streaky, oily mess. My early enthusiasm for their paints has not recovered.