Hi lemonrae,
Multi-SIM has different usage depending on your need.
For example - You have a Samsung S4 for your normal work day and a Nokia 3310 when you are in the army/overseas and you don't want to carry an expensive phone.
Instead of switching the SIM cards around that may potentially damage the SIM's chip, you can have 2 SIM cards in both the phones.
At any one time, only 1 phone is active (receive calls, SMS, whatsapp) and after keying a command into the S4 (varies from telco), it will divert the receiving to the Nokia phone.
But both the phones can be used to send outgoing messages and calls.
Like what Yo2020 mentioned, it was used to share the data bundle with another tablet device when it was still 12GB instead of signing up for mobile broadband.
You can refer to your telco for the charges and how they will sell you "multi-SIM" =)
Singtel
Multi-SIM | SingTel
M1
M1
StarHub
StarHub - Business - Mobile - Value-added Services - Optional Value-Added Services - MultiSIM