Lemon zest, the yellow part of the peel – not the white bitter part – holds the essential oils of the lemon and is thus filled with pure lemon flavor. Lemon juice, on the other hand, has the acidic, tart taste of lemon. Both have their places in cooking.
By the way, this is true of the zest of any citrus fruit.
I like lemon juice on cooked fish or steak because it adds a citrusy kick and enlivens the palate. Plus it helps with digestion. I will include the zest in dessert recipes or in my quinoa salads when I want to add more lemon flavor without overpowering the dish with acidity. Imagine you wanted to make lemon candy that wasn’t tart, you’d use the zest. If you wanted to add a pop of acidity to sweet peaches or to roasted vegetables, you’d use lemon juice.