The vitreous is a gel like substance that fills most of the eye. It is critical during the development of the eye. After the eye is developed the vitreous simply fills space inside of the eye. 
The vitreous is often the cause of many problems in the eye. If the vitreous tugs on the retina and it can cause a retinal tear. This happens when the vitreous detaches (Posterior Vitreous Detachment) or when the eye is injured.
Epiretinal membrane or cellophane maculopathy are microscopic types of vitreous traction on the macula. An epiretinal membrane can distort the vision.
Floaters are the most common annoyance caused by the vitreous. For some people floaters impair the vision by causing spots or blur.
For severe retinal/vitreal complications the vitreous is removed (vitrectomy). Removing the vitreous is not without risk and is often performed as part of other retinal procedures.