Everyone eventually gets cataracts if they live long enough. It is part of aging, much like getting gray hairs. Aging is the most common cause of cataracts. Other causes include medical conditions such as diabetes and medications such as certain steroids. Some people are born with cataracts, and some develop a cataract after trauma.
Different people have different visual needs.
If you are visually limited from performing your daily activities from cataracts, then you may be a candidate for cataract surgery. Leaving a cataract in the eye is not dangerous except for rare situations. Cataracts in general do not harm the eye. In most cases, surgery can be postponed for as long as the patient desires or not done at all. The only side-effect would be the bluffed vision from the cataract.