You probably mean the relay servers. They are used in case your devices are behind a NAT and don’t have any ports forwarded. In that case, it would be impossible for devices on different networks to communicate over the internet, so the relay servers make the communication work. It’s not something to worry, since your data is encrypted when passing through it, but it can be disabled.
It’s nice to see syncthing becoming more popular. It’s a great foss app that deserves more attention and can be used in so many contexts, replacing several proprietary and restrictive solutions