Before revision:  In large-scale optimization, when either forming or storing Hessian matrices are prohibitively expensive, quasi-Newton methods are often  used in lieu of Newton's method because they make use of first-order information to approximate the true Hessian.
After revision:  In large-scale optimization, when either forming or storing Hessian matrices are prohibitively expensive, quasi-Newton methods are often used in lieu of Newton's method because they only require first-order information to approximate the true Hessian.