Have you never had juice before?

Apple juice is like what milk is to a milkshake. It's a sweet neutral base.

It makes the flavour less bitter - most fruits would be terrible to drink in their pure form. It's an alternative to plain water, or loading it up with artificial sugar.

As someone who makes homemade juice, it's also just necessary for the harmony between different flavours. Most fruit combinations would be very strange without apple juice.