ssl certificate – Mqtt over TLS: Reasons for using self signed cert?

I have come across various mqtt over tls howto’s over the internet and for some reason every single one of them is using a self signed broker certificate. is there a reason for this?

Would it not be better to create a csr and send it to a trusted CA to sign? Does self signing have any advantages in terms of security?

Some links that I came across for mqtt over tls certs:

medium.com/gravio-edge-iot-platform/how-to-set-up-a-mosquitto-mqtt-broker-securely-using-client-certificates-82b2aaaef9c8

forums.raspberrypi.com/viewtopic.php?t=287326

Additionally, I’d have to keep a local copy of a broker cert if it is self-signed, but if it is signed by a trusted CA, I could directly connect to the broker (based off /etc/ssl/certs). Is that a correct statement?