I hear this question a lot - normally from non-nerds ;-)
The simple answer would be - because we can. Although not very satisfactory, it's pretty much it. My personal motivation was my occupational situation at that time: I love computer networks, but I had to code a lot of PHP. I like coding, but it can get pretty dreary after a while. Long story short: I was a little bored and started inspiring my friends to build a private network to experiment with VPN and dynamic routing protocols on MikroTik's RouterBoards.
So we met several times, I explained the vision of a network to experiment with and did some workshops with my friends so they could gain the skills necessary to configure the routers accordingly. We covered a whole lot of topics. We begun with the basic configuration of the MikroTik routers, moved along to firewalling, setting up static routes, function of VPN and later on we covered dynamic routing in general and OSPF in particular.
I do not know when exactly we had the first two VPN tunnels up and running, but I think it must have been around september 2014.
The core of the jumpNet network is a star. Some rays of the star are connected to further networks. The following diagram will show it:
The routers participating in the jumpNet (all the routers except 172.16.2.2 in the diagram) connect to the PowerLanJens router. On each router runs an OSPF instance to distribute the routes to the different networks.
Jens has a static public IPv4 address to ensure a decent setup without the need to automatically reconfigure the others routers with the current IP of Jens.
Firewall rules imposed on each router ensure only desirable traffic to the different networks.
At the beginning the network's character was fully experimental. Now, it is used for VoiP telephony. Friends moving from Freiburg to different cities only ordered internet access but no phone. Instead, they use their jumpNet access to connect to my astrisk pbx (see image below) and use one of my phone numbers (I got 10).
The project now is in a sleepy state. The network is still running and functional, but apart from beeing used for a little VoIP telephony, I think I'm currently the only one testing stuff (like MikroTiks EoIP (ethernet over IP) tunnels) ;-)
After creating the jumpNet, I learned, that there is a project with similar goals: DN42
Maybe it would be fun to connect the jumpNet with DN42...but this is another story.