Network selection is more for roaming purposes. Essentially your phone can "see" other network operators broadcasting in your area. Your phone chooses a network to join, in your case its t-mobile because thats the sim card you have. If your t-mobile signal disappears, your phone will attempt to connect to the other 3 carriers and will connect if tmobile has a roaming agreement with them. The network selection comes in more handy when you are in another country with multiple roaming partners. If carrier A has better coverage/speeds than carrier B, you can manually select which carrier to prefer to connect to while roaming.