TTN is a lot of things, so everyone here will have different reasons for using it or being a part of it.
TTN develops an implementation of a LoRaWAN network server. There are other (free, open source) implementations, as @azazeal wrote, and each has a specific goal (run on the gateway, private networks, development, …). Our v2 implementation works really well for world-scale distributed networks like TTN, and our upcoming v3 stack will allow you to start with a tiny development network (on your RPi gateway) and grow into a highly available, distributed, worldwide enterprise network. Using TTN’s network server means that a network can grow to TTN scale. We’ve already proven that it works and we have companies providing enterprise networks with support and SLAs. Of course the beauty of LoRaWAN is that it’s open, so you are free to choose a different network server if that works better for you.
TTN provides a worldwide LoRaWAN network. This network allows anyone to connect their gateways, register devices and send/receive data for free. The servers are hosted by sponsors such as The Things Industries, Meshed, Switch and DigitalCatapult. You also can set up your own, private LoRaWAN network, or use one provided by your national telecom operator. I think the biggest advantage of TTN is that you can use existing coverage without any investment, and add coverage when you need more.
TTN is 50 000+ developers gathered in 600+ communities in 100+ countries. Those communities bring people together: developers, businesses, governments work together on building cool things and on solving (local) problems.
TTN is probably the best source of LoRaWAN knowledge out there. With the shared knowledge of our “Learn” section, our “Labs” stories and this forum we are on Google’s first page for any LoRaWAN question you may have. If there’s no answer, you can ask the community here on the forum, or on Slack.
I would actually be very interested to hear the reasons people may have for not using TTN…