MQTT or serial communication

[MagnusF]

I'm a newbie and have just started a project to control things in my home.
To learn, I have set up OH2 on a raspberry pi 3, using an Arduino nano as serial gateway and two nodes.
Everything works well. Now I have read here in the forum about MQTT which many seem to use but not found when or why to use it.
Hope anyone can explain the benefits of the two methods.
Are there situations when MQTT has to be used?
If using a MQTTgateway, as described in MS, do I have to have a MQTT Broker e.g. on a RPi?

[gohan]

Higher security if you have incoming connections from outside your Lan and more flexibility if you have other devices accessing data directly on mqtt, but if you are running just your simple setup there is no real need