To understand the function of the circulatory system it is best to first reflect on simple unicellular organisms floating in a pond or in a sea. The pond functions to deliver nutrients and oxygen to the unicellular organism. When you have a multicellular organism that needs to move about, the multi-cellular organism carries the pond around with it. Special organs are needed to put oxygen into the pond and to remove carbon dioxide and other harmful metabolites. So a circulatory system functions to move the pond about the body to the various specialised organs and to the tissues that need the services that the pond provides. Unfortunatley with this increased complexity there are new problems related to malfunction of the component parts.
The difference between and end artery and other arteries is that the end artery is the only artery supplying the tissue that it supplies. When the end artery is blocked there is no opportunity for the distal tissues to recover their blood supply from other arteries. Examples of end arteries are the retinal artery and the arteries beyond the circle of Willis.