When I send an email to somebody, (I think) my email is sent to my home server, then the email is sent to target server, then it is finally sent to the people I want to send to.
Sometimes, this needs multiple hops, which is the point that confuses me. Why does this need multiple hops? Why can't the email be sent to the target server directly? XMPP doesn't need multiple hops, for example.
Fundamentally that has to do with the original purpose of the Internet, or rather it's predecessor ARPANET, was designed to survive the loss of portions of the network (popular to popular misconception, this design goal existed not in order to survive nuclear attack, but because network components were rather unreliable at the time).
The packet switching network is designed to find one or more routes from where the packet starts (your email client) to where it needs to be (the email client of the person you are sending to).
In order to achieve that flexibility, the system is designed not just to route directly from point A to point B, but to find an available route (from a possibly very large set of possible routes) that is currently functioning.
If your email client had to know how to directly send to every other device in the world, without intervening subnets managing traffic, your computer would need a direct connection to every other computer in the world. That would be prohibitively expensive. Instead, routers inbetween accept packets from you and other people in your area and figure out how to route each packet closer to its destination. That's similar to the method that the airlines use to have regional flights feed hubs.