Saving money by using different carriers in different regions and having cell phone coverage across the globe are two advantages to dual-SIM cellphones. Dual-SIM cellphones support two SIM cards at once, effectively putting two phones inside of a single device.
Each SIM card in a dual-SIM phone can run on a different network and phone number, which is supported natively by the phone's software. If a particular network offers a reduced cost in one region, the phone can be used on that network when it's financially advantageous. Similarly, if a particular network does not offer coverage in a particular region, but another network does, the dual-SIM phone can automatically use whichever network is supported.