configure SNMP Trap monitoring in Nagios

I am struggling a lot to understand and setup SNMP Trap monitoring using Nagios monitoring tool. I followed many tutorials and articles over internet but unable to understand below 2 points:

1) Understanding about SNMP Trap monitoring, why we do SNMP trap monitoring ? 2) How do we implement SNMP Trap monitoring using Nagios monitoring tool ?

Any help on above 2 points would be highly appreciated.

Thanks in Advance.

Copyright Notice：Content Author:「Jonus Joseph」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/39120510/configure-snmp-trap-monitoring-in-nagios