Is baking soda necessary when making chocolate chip cookies?

Yes, or else you risk very flat cookies. Both baking soda and baking powder are leavening agents that produce carbon dioxide, which makes baked goods rise. Baking soda creates carbon dioxide bubbles instantly when it's mixed with acidic ingredients such as buttermllk, sour cream, brown sugar, or lemon juice. Because the soda and acid begin to react as soon as a liquid is added, any recipe using baking soda should be baked immediately, before all of those bubbles deflate.