There may be a basic flaw in my understanding of how the internet works so sorry if this is a simple question. What type of attack does VPN protect against? For example if a local area network (let's call it alpha) needs to connect to local area network (let's call it beta) that is in a different city, often VPN is used. But why? Let's say it wasn't. Then packets are sent from alpha, they go through alpha's router to the ISP, the ISP sends the to beta's router, and then beta has them. I assume the ISP is fairly secure, for example a person can't easily sniff packets inside the ISP. So at what point does having a VPN that encrypts packets actually become useful? Is it that after packet's leave alpha's router they actually pass through a series of public server before they reach the ISP or something like that?
EDIT: let me reword the question. IP suite guarantees that a packet with destination address xxx.xxx.xxx.xxx will be delivered to xxx.xxx.xxx.xxx. Obviously if the sender or receiver is compromised by a hacker then the hacker will know the contents of the packet but the point of VPN is to secure the packet is in transit. But where in transit is it week without encryption? For example even if a hacker knows the IP address of a router that the packet will pass through he can't trick the router into giving him the packet, or can he? Is the argument we don't know who owns the router so it could have been tempered with so the owner is sent a copy of every packet?
Basically I'm saying if IP already sends it to one specific recipient, why is VPN needed?