When you buy an iPhone from Smart Philippines carrier, your phone is locked to that SIM cards/Network, that means you can only use Smart Philippines service. Unlocking your iPhone will allow you to bypass that lock and use it with the carrier of your choice. You can easily switch SIM cards using the same phone. Unlocking your iPhone phone has many advantages such as allowing you to use it with another Network of your choice or even use it with a local network while traveling saving you hundreds of roaming charges.
About Smart Philippines carrier
Smart Communications, Inc., commonly referred to as Smart, is a wholly owned wireless communications and digital services subsidiary of PLDT, Inc., the Philippines' leading telecommunications and digital services provider. The company has 61.8 million subscribers on its GSM network as of end-September 2016, under the brands Smart, Sun, and TNT. Smart's wireless broadband subscribers number 3.8 million under the brands Smart Bro and Sun Wireless Broadband. (Wikipedia)