A very basic question about PLC field input switches
The normally-open and normally-closed contact instructions you see in the logic do not correspond in any way to what type of switch is wired to the input. When writing the logic, you choose the instruction based on what you need to make the logic work correctly. In fact, you often have the same input used multiple times in the logic, using a normally-open instruction in some places and normally-closed in others. 

The choice of what type of physical hardware switch to use is based on what the fail condition would be if a wire breaks or is disconnected. For example, a machine start push button should be a normally-open switch. If the connecting wire is disconnected, you will be unable to start the machine, which is safe. If you had used a normally-closed switch there and the wire broke, the machine would start by itself (unsafe). 

Likewise, a machine stop push button should be a normally-closed switch. A wire break in that case would stop the machine (safe).  If you incorrectly use a normally-open switch here, then a wire break would leave you unable to stop the machine (unsafe).