It’s not essential but this is one of Unity’s revenue sources, so they will advertise it . In fact, one key aspect to finding success (disclosure: I am an ex Unity AssetStore publisher) in the AssetStore is to persuade buyers in thinking your asset is essential for making their project come true. It’s all about selling shovels and dreams.

Where is it officially recommended? There is other content that is created by Unity but that is clearly labeled as such.

Nothing from the Asset Store is essential, but it would take much longer to do everything yourself from scratch, as well as being difficult to learn how to do everything yourself without examples to experiment with.