gas - Dryer calls for 1/8 NPT plug - why not just direct

Any ideas why this dryer gas connection diagram calls for the "1/8" NPT Pipe Plug" - why isn't it just calling for a direct connection to the gas supply instead of having something t

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Suitable for the outer diameter of the pipe:1/4 5/16 3/8 1/2 . Product Material: Brass. Used with plastic and copper tubing. Package Include:60PCS

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