Sync files (KeePass) between devices (PC, mobile, tablet, RPi)
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.