Sparse matrices are more memory-efficient than dense matrices when dealing with large matrices that have a significant number of zero elements. By storing only the non-zero elements explicitly, sparse matrices can save memory and computation time for certain operations. However, there are situations where operations or algorithms may require dense matrices. For example, some mathematical operations, such as matrix multiplication or certain linear algebra operations, may be more efficient or easier to implement with dense matrices. Dense matrices allow for straightforward element-wise operations and can leverage optimized libraries for linear algebra computations.