Your behavior is even stranger than that which you are questioning. The fact you take the time to type out the response means you also feel the urge to respond, and that urge is strong enough that you don't stop yourself before starting, even though you know you are likely to just delete before sending. It's odd that you would ask why someone would reply to a comment when you should already understand why someone would do that: because they feel the same urge you do, and don't have the compulsion to stop themselves before sending.