Learn Tai Chi Maker - mqtt Chapter 2 (3) reserved messages

Video link ：https://www.bilibili.com/video/BV1pv411b7o1/?spm_id_from=autoNext&vd_source=b91967c499b23106586d7aa35af46413
The data link ：http://www.taichi-maker.com/homepage/esp8266-nodemcu-iot/iot-tuttorial/mqtt-tutorial/retained-messages/

“ Keep the message ” Is very important MQTT Concept . adopt “ Keep the message ” The name is not difficult to judge ,“ Keep the message ” It's a message that's been preserved . But this “ Keep the message ” Why should it be kept ？ And what is the special purpose of keeping messages ？