Simply because you can only add so much lemon zest to a cake before the tough zest starts to compromise the texture. This cake calls for a whopping two tablespoons of zest, but Rose still wanted more lemon flavor, hence the oil. She says she uses lemon oils over bottled extracts but “there’s lemon oil and there’s lemon oil.” Rose made a point to specifically recommend Boyajian lemon oil, which she says it offers the purest essence of lemon she’s ever tasted. I’ve seen that particular brand on sale at Williams-Sonoma. Failing that, look for flavored oils at cake decorating and/or candy making supply stores. It comes in really tiny bottles.
Also, for recipes that call for lemon zest, Rose strongly suggests washing the lemon in hot water and detergent — not soap — before zesting it. The best product to use? Liquid Dawn, which removes residues on the surface of the lemon that impart bitter flavors.