Why dont we use Given's method to reduce a symmetric matrix to diagonal form rather than tridiagonal form, and why is tridiagonal form desirable?

Question

I've been searching for a while trying to understand why we use Givens method (and householders method) to reduce a symmetric matrix to tri-diagonal form instead of using these methods to reduce a matrix to diagonal form, similar to the QR decomposition. Since surely diagonal form is more desirable, and on this note, why would it be desirable to reduce a matrix to tri-diagonal form. Thank you for any help anyone can give towards helping me understand this small point.