What will an mri with contrast reveal about a knee injury that won't show with out contrast?

MRI: I'm assuming you mean intra-articular contrast (arthrogeam). It adds additional detail making more subtle injuries more clear (ie small cartilage tears, cartilage erosion, etc). Simply put...Contrast adds clarity. Even more useful for shoulders.

If prev surgery: Contrast injection into the knee (arthrogram) is helpful if you have had previous meniscal surgery.