The meniscus is a very important structure. It acts as a shock-absorber which reduces the pressure that is transmitted through your knee.
Your knee will wear out quicker if your meniscus is injured because more pressure is being put through your knee. This increased pressure can eventually lead to increased wearing out of your cartilage and osteoarthritis.
The injured meniscus can flip in your knee joint, causing a clicking sensation when the knee is bent. It can also lead to locking of your knee. Also, a torn meniscus can result in persistent knee pain.