Have you ever wondered why anyone would need a phone with dual SIM capabilities? I did too until recently. It's all about coverage and being in contact. Here in Australia Telstra has the best coverage, but there are places where the Optus network has better reception. Here at my home is one example where Optus is best, but when travelling the Telstra network generally gives better coverage.
A dual-SIM phone gives me the best of both worlds. I have my main SIM with Optus on a Prepaid $30 Monthly+, which gives me unlimited calls & texts, 1.5GB data and $5 credit for overseas calls etc. My secondary SIM is with AldiMobile PAYG using the Telstra network ($15 top up with 365 days credit). I have set my Optus SIM to divert to my AldiMobile SIM when my Optus number is unobtainable, so as long as I am within range of either an Optus or a Telstra tower, I can receive calls.
To improve mobile use where WiFi is available, Optus have just introduced an app called WiFi Talk. This enables an Optus customer to make and receive calls and SMS to their Optus number via WiFi (call costs same as for cellular network), great for inside a building where the signal strength can be weak or non-existent. I now use it at home, it saves me wandering around near windows looking for a decent signal. With dual-SIM and WiFi I have maximised the useful coverage for my phone.