You receive the stream through your ISP and through whatever route that connection takes from the server to the ISP and from the ISP to you. If there is a bottleneck in any portion of that journey, there can be issues. Also, if (and it is only an if) your ISP doesn't allow traffic to or from the server, a VPN masks information to and from the server, allowing the stream to pass unnoticed/unrestricted. Moreover, if there is a bottleneck in the route, a VPN can channel the information through a different (often more direct) route, bypassing whatever bottleneck existed.