If your router still works I wouldn't bother with buying a new one. If it breaks however it's almost always cheaper to buy a new one through a company like Amazon or Best Buy than to replace it through your ISP. For example, when my motorola modem went, AT&T wanted $150 to replace it. I could have bought the same modem from Best Buy for $75.
1- because your ISP usually "rents" it to you, secondly, having your own, give you control over your LAN, and it adds a layer of security. If you connection is compromised, only the ISP router would be at risk, nothing past that would come to harm